[Chapel Merge] Remove outdated reference to llvm@7 in llvm techno

Branch: refs/heads/master
Revision: 539c654
Author: ronawho
Log Message:

Merge pull request #16720 from ronawho/llvm-docs

Remove outdated reference to llvm@7 in llvm technote

We no longer support llvm 7 and brew installs don’t work at the moment
(Cray/chapel-private#1294) so just remove the docs about it.

Modified Files:
M doc/rst/technotes/llvm.rst

Compare: https://github.com/chapel-lang/chapel/compare/94420007bada...539c654f6d3b