Remove some old patches for things that now compile with 8.8/HEAD

Each of these libraries now has a newer Hackage release that compiles with GHC 8.8/HEAD. This induces some knock-on changes due to pandoc now having a different build plan:

• Add a patch for regex-pcre, since pandoc now builds that instead of regex-pcre-builtin
• Add a Nix dependency on the C pcre library so that regex-pcre builds successfully