Remove ancient threads diagnostics from perldiag

for several reasons:
• The wording has changed, so nobody is using these entries anyway.
• The perldiag entries are clearly not being maintained.
• The threads module has its own list of diagnostics.
• The actual wording depends on the version of the threads module
  installed, not the version of perl.
toke.c:S_tokeq: remove dead code

The argument passed to tokeq is always a plain string owning its own
buffer.  I checked all the callers.

This code has been like this since perl 5.000.
toke.c:S_tokeq: add comment
Move threads::shared warnings from perldiag to shared.pm

following the precedent set by threads.pm.
Increase $threads::shared::VERSION to 1.45