configure: Remove redundant pkg-config code
configure: Remove redundant pkg-config code

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.

bitcoin/secp256k1,bitcoin/secp256k1,peterdettman/secp256k1,bitcoin-core/secp256k1,peterdettman/secp256k1,bitcoin/secp256k1,bitcoin/secp256k1,ElementsProject/secp256k1-zkp,ElementsProject/secp256k1-zkp,bitcoin/secp256k1,bitcoin-core/secp256k1