games/chessx: Improve port

• After remove ".if ${COMPILER_TYPE} == clang":
  • remove ".include <bsd.port.pre.mk>";
  • replace ".include <bsd.port.post.mk>" with ".include <bsd.port.mk>".
• Remove the useless block from the patch that adds one blank line.

PR: 292841
Approved by: Alexey Vyskubov <alexey@pentode.fi> (maintainer, timeout 2 weeks)