Balay/petsc remove boost dependency (#8508)

* petsc: remove dependency on boost - its required only as a dependency on trilinos

Also set COPTFLAGS='' etc - this way CFLAGS is the primary way optimization flags are set by spack

* petsc: update pkgs with petsc~boost dependency

* fix flake8

* boost is optional for trilinos - so adjust petsc dependency on boost [via trilinos] accordingly

* do not add boost dependency on petsc [due to trilinos requirement]. Trilinos adds this dependency anyway.