bump-formula-pr: fix removal of old mirrors

Previously, old mirrors are only removed if the requested spec is
stable, and if the mirror lines only have two leading spaces. This leads
to stale mirror line(s) when the formula a stable block like

    stable do
      url "http://example.com/v1.0.tar.gz"
      mirror "http://example.net/v1.0.tar.gz"
    end

where the mirror line is lead by four spaces.

In this commit, we discard the /(^ mirror .*\n)?/ pattern, and instead
create a pattern with the exact url and flexible leading spaces for each
mirror of the requested spec.