Remove bogus tag service-key-has-whitespace. (Closes: #946763)

This key is either bogus or has become outdated. According to a
clarification from systemd's upstream from February 2019, such spaces
are now safely ignored:

    https://github.com/systemd/systemd/commit/170342c90be07f418ab786718d95ef76289126a0

That will presumably prevent the third-party
tools mentioned in the tag description from breaking.

Removes tag definition and the relevant check code.