[perl #121771] Revert the new warning for ++ on non- /\A[a-zA-Z]+[0-9]*\z/

This failed as in it was producing:

  Argument "123abc" treated as 0 in increment (++) at -e line 1.

when the user incremented that value (which is a lie).

This reverts commits 8140a7a801e37d147db0e5a8d89551d9d77666e0 and
2cd5095e471e1d84dc9e0b79900ebfd66aabc909.

I expect to revert this commit, and add fixes, after 5.20 is released.

Conflicts:
	pod/perldiag.pod