gdb: remove return value of set_language

set_language returns the previous language, but nothing uses it.  Remove
the return value.  This lets us remove the assignment to
current_language, in _initialize_language.

Change-Id: Ifccf9b488434c1addf4626130a74e159a37d8c17
Reviewed-By: Tom Tromey <tom@tromey.com>