sysutils/htop: simplify Makefile

This change cleans up the Makefile to follow Porter's Handbook guidance.
Here is the rationale for each change:

• As per https://docs.freebsd.org/en/books/porters-handbook/book/#makefile-master_sites-github, ports are urged to move to using Github releases instead of git tags. This change replaces USE_GITHUB with MASTER_SITES to statically point to htop release tarballs. This also requires the addition of USES=tar:xz
• Remove USES=autoreconf because the release tarball doesn't need to run it
• Remove USES=compiler:c11 since upstream https://github.com/htop-dev/htop states that a C99 compiler is needed.
• Add USES=desktop-file-utils as the port installs htop.desktop
• htop does not exist in the official NVD CPE dictionary, so as per https://docs.freebsd.org/en/books/porters-handbook/book/#uses-cpe, we should "not ever make up CPE data." Thus, USES=cpe is removed until an entry is contributed to the CPE dictionary
• USES={libtool,localbase,pkgconfig,python:build,shebangfix} are not necessary for a successful build and run with the GH release tarball.
• GNU_CONFIGURE_MANPREFIX was unnecessary since default value in Mk/bsd.port.mk is same ${PREFIX}/share

PR: 280820
Approved by: Hung-Yi Chen <gaod@hychen.org> (maintainer)