autoconf: delete obsolete unused m4 file

config/uintmax_t.m4 contains only one function: jm_AC_TYPE_UINTMAX_T.
After a grep, I could not find any usage of it.

jm_AC_TYPE_UINTMAX_T seems to be an old implementation of the officially
suppported AC_TYPE_UINTMAX_T.
Doc: https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/
     autoconf-2.72/autoconf.html#index-AC_005fTYPE_005fUINTMAX_005fT-1

config/stdint.m4 seems to have taken over. The usage of
AC_REQUIRE([AC_TYPE_UINTMAX_T]) is not garded or inside a function, so it
will also automatically be run for the configure.ac files using
AC_CONFIG_MACRO_DIRS([../config]) instead of m4_include([../config/stdint.m4]).

It seems that people replaced jm_AC_TYPE_UINTMAX_T occurences by
AC_TYPE_UINTMAX_T, and forgot to delete uintmax_t.m4.
Deleting the file and regenerating the build scripts results in no diff,
so it looks safe to delete it from the repository.