Cleanup dependencies and license checks

[franziskuskiefer]

After adding license checks in #1035 I reviewed them in #1088. This issue capture the resulting action items.
The licenses themselves are ok. But the checking isn't quite right and there's a dependency that needs to go.

Remove depdencies

Drop proc-macro-error. It is unmaintained. There's an alternative crate (proc-macro-error2) that may be an alternative if needed.

Script does not work

The script to check licenses doesn't work (at least on macos). grep: invalid option -- P.
The resulting bigger problem is that it still print "ok" even when it didn't do anything.

[W95Psp]

Ah, that's the common grep MacOS issues...