buildRubyGem: remove Makefiles
The generated Makefile is not reproducible. The HDRS section is inode
dependent. "sitearchdir" and "sitelibdir" contains a timestamp and
random postfix. See https://r13y.com/diff/8350ec80865096f84705f3e000d97f151760e768bf626e52c84570413c5e1093-f9dea788fe62d683e90e12e166f29b778c39c699a5be39f9262e5cffb77d3e45.html
Instead of trying to make it reproducible, remove it entirely. It was
only useful for generating the .so file and it no longer needed.
See also https://bugs.ruby-lang.org/issues/15304
Fixes the rubyPackages.nokogiry reproducibility for example.
c3357d2cd9