refactor(exe_name): remove unnecessary ".exe" suffix

[mwtoews]

This PR removes .exe suffix from various exe names in the main flopy module and in examples. There are a few reasons for this:

• The .exe suffix unnecessary for Windows, since executables are defined as any file with suffixes listed in PATHEXT. Calling mf2005 in Windows will automatically find mf2005.EXE if it is in one of the folders from the PATH environment variable.
• It is inconsistent for model classes and load methods to have defaults (e.g.) exe_name="mf2005.exe" that imply they are designed only for Windows, when these same classes and methods work cross-platform for Linux and macOS too.

Note that these changes should not break anything downstream, e.g. someone with a script with exe_name="mf2005.exe" will still continue to work on Linux/macOS (mbase.py will remove .exe if the executable is not found).

Other changes:

• Change BaseModel.exe_name from mf2k.exe to mf2005, although this is an abstract base class, so this modification does not change anything.
• Clean-up several import platform statements where the module is not used.

[codecov]

Codecov Report

Merging #1563 (31a2dc1) into develop (14d69cb) will increase coverage by 0.0%.
The diff coverage is 0.0%.

@@           Coverage Diff           @@
##           develop   #1563   +/-   ##
=======================================
  Coverage     72.4%   72.4%           
=======================================
  Files          251     251           
  Lines        54216   54215    -1     
=======================================
+ Hits         39300   39303    +3     
+ Misses       14916   14912    -4     

Impacted Files	Coverage Δ
flopy/mbase.py	68.8% <0.0%> (+0.3%)	⬆️
flopy/mf6/mfmodel.py	56.5% <ø> (ø)
flopy/mf6/modflow/mfgwf.py	100.0% <ø> (ø)
flopy/mf6/modflow/mfgwt.py	76.0% <ø> (ø)
flopy/mf6/modflow/mfsimulation.py	58.7% <ø> (ø)
flopy/mf6/utils/createpackages.py	7.4% <0.0%> (ø)
flopy/mfusg/mfusg.py	77.4% <ø> (ø)
flopy/modflow/mf.py	68.4% <ø> (ø)
flopy/modflowlgr/mflgr.py	52.5% <ø> (ø)
flopy/modpath/mp6.py	83.9% <ø> (ø)
... and 3 more

[christianlangevin]

Long overdue. Two thumbs up.