This removes code that detects the pkg-config tool. We used this
back in the days when we had dependencies. ;) It can always be brought
back if we'll need it in the future.

Note that we still deliver a .pc file for this library, and there is
code in Makefile.am to install it. But this does not require the
pkg-config tool; only consumers of the .pc file will need it. This can
be verified by running make install (maybe after mkdir /tmp/pre and
./configure --prefix=/tmp/pre and checking that the .pc file is
installed correctly.

This is a backport of secp256k1#1090