Remove STRING_BUFFER() macro in gssapi_generic.c

Remove STRING_BUFFER() macro in gssapi_generic.c

In gssapi_generic.c, struct mech_attr_info_desc included some
gss_buffer_desc members whose length fields were never used.
Additionally, the STRING_BUFFER() macro's computation of the (unused)
length fields was incorrect, causing warnings in some versions of
clang.  Remove the problematic STRING_BUFFER() macro and adjust the
array and generic_gss_display_mech_attr() accordingly.