OpenSSL: install .pc files from the exporters subdir

The .pc files generated in the root directory are used as part of the
build; they should never be installed. Use the versions from the
exporters subdirectory--which should be installed--as the .pc files
which are distributed with FreeBSD. This avoids the need for "fixing up"
these files after the fact (see crypto/openssl/BSDmakefile for more
details as part of this change).

Garbage collect secure/lib/libcrypto/Makefile.version, et al,
as they're orphaned files. They were technically unused prior to this
change as the vendor process properly embeds the version numbers in
various files, but this commit formalizes the removal.

This correction/clarification on the .pc files will be made in an
upcoming release of OpenSSL [1].

References:

1. https://github.com/openssl/openssl/issues/28803

Suggested by: Richard Levitte (OpenSSL project)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D53043

(cherry picked from commit 97388e7215e080f2cb9fd446f4be4e46a9aeb114)