bug 1171957 - Drop redundant spec tests
bug 1171957 - Drop redundant spec tests

Drop content parsing spec tests, but retain those that exercise the
visitors, to stay at 100% coverage.

mdn/browsercompat,jwhitlock/web-platform-compat,renoirb/browsercompat,renoirb/browsercompat,jwhitlock/web-platform-compat,jwhitlock/web-platform-compat,mdn/browsercompat,mdn/browsercompat,renoirb/browsercompat