CPack/RPM: Remove superfluous %clean section from generated spec file

According to [this] commit message in the `rpmlint` codebase,
a %clean section is not necessary since `rpm` 4.8.0 (2010)
and nowadays `rpmlint` at least issues a warning about it, and
(seems) in some distros it's even considered as an error.

[this]: https://github.com/rpm-software-management/rpmlint/commit/b1a8a422

Modules/Internal/CPack/CPackRPM.cmake

@@ -1747,8 +1747,6 @@ Vendor:         \@CPACK_RPM_PACKAGE_VENDOR\@
 %install
 \@TMP_RPM_INSTALL\@
 
-%clean
-
 %changelog
 \@CPACK_RPM_SPEC_CHANGELOG\@
 "
@@ -1890,8 +1888,6 @@ mv %_topdir/tmpBBroot $RPM_BUILD_ROOT
 
 \@TMP_RPM_DEBUGINFO_INSTALL\@
 
-%clean
-
 \@post_\@
 \@posttrans_\@
 \@postun_\@