gdb: do not recompute values for inferior parameters when showing them

gdb: do not recompute values for inferior parameters when showing them

When calling the `show` command on a few inferior-specific settings
(inferior-tty, args, cwd), GDB will recompute the values despite them
being provided by a getter.

For instance, When `show cwd` is called, `do_show_command` will query
the value of the `cwd` through `get_inferior_cwd`, which already gets
the per-inferior value. The current version seems to be a workaround
from when settings could not provide a getter function and per-inferior
settings could thus not be implemented properly, forcing the `show`
implementation to recompute the value itself.

The changes clean up the code and makes the show command trust the
values it is being forwarded, as they will always be computed by the
getter function.

Co-Authored-By: Simon Marchi <simon.marchi@efficios.com>
Change-Id: I3b25f61f9101d98a6df7d50cee50131aec7e25c9
Approved-By: Andrew Burgess <aburgess@redhat.com>