gdb: remove make_function_type

We now have make_function_type and lookup_function_type exposed by
gdbtypes.h, which do essentially the same thing.  Remove
make_function_type, inlining its code inside create_function_type.
Change all other callers of make_function_type to use
lookup_function_type instead.

Change-Id: Id7c25f02059efe5c0f15e8ab8a35ac1fa97d9d6a
Approved-By: Tom Tromey <tom@tromey.com>