security/fsh: Remove redundant escapes of ordinary characters

Makefile.in has a number of escapes (all following @ in the sed expression)
that only serve to stop a sed in the configure script from replacing them.
Escaping ordinary characters is considered undefined behavior by POSIX, and
the patch in bug #229925 aims to serve out errors for escaping of most
ordinary characters to reduce friction when some of these sequences are
given special meaning.

QA:

• portlint: OK (minor warnings that should be addressed separately)
• testport: OK (poudriere: 13.0-CURRENT and 12.0-RELEASE, amd64)
• diffoscope: OK (only minor metadata changes to package)

PR: 240308
Approved by: koobs (ports)