autoconf: remove 'deeper' checks for `AC_CHECK_FUNCS`

autoconf: remove 'deeper' checks for `AC_CHECK_FUNCS`

The net effect of the deeper checks is to raise implicit function decls
on modern compilers.

These checks appear to have been added ~20 years ago, relating to an
unverifiable claim about HP-UX. Autoconf support for the platform has
grown in leaps and bounds since.

It didn't cause a real problem here, but when investigating a FP this
came up. No evidence has been identified that this was actually broken
in the past, and there is no evidence that this is necessary now.

`-Werror=implicit-function-declarations` is enabled for both checks;
without a working prototype they will both fail regardless. In the
second case there will in fact never be a working prototype and
therefore it will always fail unconditionally.

`AC_CHECK_FUNCS` does effectively the same thing as the removed checks,
except it actually defines a dummy prototype to see if it links.

If `AC_CHECK_FUNCS` is broken on a given platform we have bigger
problems than trying to build cURL. This should also be faster.

Bug: https://bugs.gentoo.org/932827
Reviewed-By: Eli Schwartz <eschwartz93@gmail.com>
Closes #13830