Remove unused function get_writeable_symbol

[tautschnig]

The function has no user and no documentation, it's purpose is not entirely
obvious, the named parameter was not used -- a lot of code smell overall.

[tautschnig]

@NathanJPhillips could you please take a look at this?

[smowton]

It's an iterator with an explicit write operation -- i.e. default operator* and operator-> are const, but there's an explicit write operation if you want to make a change, a la lookup vs. get_writeable, which do and don't generate a journal entry in journalling_symbol_tablet.

It should definitely be documented, for which I choose @NathanJPhillips (delegation is awesome ;)); I guess its user either died or never got implemented in the end? But a non-const iterator over the symbol table doesn't seem like an unreasonable thing to want. I'm not convinced we should kill it though -- instead we should add docs, remove the needless parameter, and add it to symbol-table-base's unit tests.

[tautschnig]

[...] I'm not convinced we should kill it though [...]

I'm perfectly happy keeping it if it gets documented and cleaned up. I'll happily take all guidance from @NathanJPhillips on how to fix it.

[allredj]

Passed Diffblue compatibility checks (cbmc commit: 53a5061).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/76919827

[NathanJPhillips]

#2492