Remove Colors.plist and fix CI

CI is currently failing since :highlight auto-complete relies on loading
v:colornames implicitly, but MacVim has its own special handling and
parses a bundled Colors.plist file instead, which bypasses the whole
system.  Colors.plist was added a long time ago as Vim did not have a
good cross-platform way to specify colors by names, but this has been
alleviated upstream a while ago via runtime/colors/lists/default.vim
which gets sourced for default color values.

To fix this, simply remove the file, as it serves no purpose anymore.
Also remove custom hex color parsing logic which also should simply
defer to Vim instead.

Author: ychin

runtime/doc/gui_mac.txt

@@ -402,32 +402,23 @@ top of the screen, you can set |MMNonNativeFullScreenShowMenu| to `NO` and
 ==============================================================================
 5. Special colors					*macvim-colors*
 
-The colors in MacVim are defined in two dictionaries inside the "Resources"
-folder of the application bundle (MacVim.app/Contents/Resources).  It is
-possible to add more colors by modifying these files.  Color names are case
-insensitive when accessed from Vim, but in the dictionary they must be
-lowercase.
+MacVim mostly uses standard Vim colors.  See |gui-colors| and |v:colornames|
+for how to set and override them.
 
 							*SystemColors.plist*
-There are only a few system colors that can be accessed from Vim.  These
-colors are defined in the dictionary "SystemColors.plist".  This dictionary
-stores (key, value) pairs where the key is the name of the color and the
-value is an NSColor selector name.
-
-The most useful system colors are: >
-	MacSelectedTextBackgroundColor
-	MacSecondarySelectedColor
-The former is the "Highlight Color" which can be changed in the "Appearance"
-section of the System Settings.  The latter is the selection color used by
-a Cocoa application when it is not in focus.
-
-							*Colors.plist*
-Apart from the system colors, it is also possible to use the standard X11
-color names (see https://en.wikipedia.org/wiki/X11_color_names) which usually
-come in a file called "rgb.txt".  MacVim does not have such a file, instead it
-keeps these colors in a dictionary called "Colors.plist". The key in this
-dictionary is the name of the color and the value is an RGB value on the form
-#rrggbb stored as an integer.
+There are a few additional system colors that can be used in the |:hi|
+command.  These colors are defined in the dictionary "SystemColors.plist" in
+the MacVim.app bundle.  These color values correspond to NSColor selectors in
+macOS.  The available color names are:
+
+KEY					VALUE ~
+MacSecondarySelectedControlColor	Selection color when app is not in
+					focus.
+MacSelectedTextBackgroundColor		"Highlight Color" which can be changed
+					in the "Appearance" section of System
+					Settings.
+MacTextBackgroundColor			Normal text background color.
+MacTextColor				Normal text color.
 
 							*macvim-colorscheme*
 MacVim ships with a custom color scheme that is used instead of the default

runtime/doc/tags

@@ -4124,7 +4124,6 @@ CmdwinEnter	autocmd.txt	/*CmdwinEnter*
 CmdwinLeave	autocmd.txt	/*CmdwinLeave*
 ColorScheme	autocmd.txt	/*ColorScheme*
 ColorSchemePre	autocmd.txt	/*ColorSchemePre*
-Colors.plist	gui_mac.txt	/*Colors.plist*
 Command-line	cmdline.txt	/*Command-line*
 Command-line-mode	cmdline.txt	/*Command-line-mode*
 CompleteChanged	autocmd.txt	/*CompleteChanged*