Revert "Merge branch 'update-sassc' into 'master'"

This reverts !27267 (merged) because it fails to build on CentOS 6:

 The following shell command exited with status 1:
    $ CC=/opt/rh/devtoolset-6/root/usr/bin/gcc CPP=/opt/rh/devtoolset-6/root/usr/bin/cpp CXX=/opt/rh/devtoolset-6/root/usr/bin/c++ /opt/gitlab/embedded/bin/gem install sassc --version 2.2.1
Output:
    Building native extensions. This could take a while...
Error:
    /opt/gitlab/embedded/lib/ruby/site_ruby/2.6.0/rubygems/ext/builder.rb:76: warning: Insecure world writable dir /builds/gitlab/omnibus-gitlab in PATH, mode 040777
ERROR:  Error installing sassc:
	ERROR: Failed to build gem native extension.
    current directory: /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/sassc-2.2.1/ext
/opt/gitlab/embedded/bin/ruby -r ./siteconf20200315-29430-15yeaq8.rb extconf.rb
creating Makefile
current directory: /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/sassc-2.2.1/ext
make "DESTDIR=" clean
current directory: /opt/gitlab/embedded/lib/ruby/gems/2.6.0/gems/sassc-2.2.1/ext
make "DESTDIR="
compiling ./libsass/src/ast.cpp
cc1plus: error: unrecognized command line option "-std=c++11"
cc1plus: error: unrecognized command line option "-flto"
make: *** [ast.o] Error 1
make failed, exit code 2

This was added in https://github.com/sass/sassc/blob/c8f524c6c1819adee77d3d16a36f0ccdffab1676/Makefile#L79.