Omit the `omake` compilation-mode rule by default

Omit the `omake` compilation-mode rule by default

It keeps interfering with other rules, slowing everything down a
little bit and makes it harder to add or change other rules.  The rule
is still there and can easily be re-enabled by those who need it.

* etc/NEWS: Announce.
* lisp/progmodes/compile.el (compilation-error-regexp-alist):
Exclude `omake`.
* test/lisp/progmodes/compile-tests.el
(compile-tests--test-regexps-data):
Actually test the `cucumber` rule. Remove the `omake` test case.
(compile-test-error-regexps):
Test `omake` here.  Test other rules without `omake` included.