CC Mode: Remove unused code from c-maybe-re-mark-raw-string

Acknowledge that when the function is called, it is always at the start of a
string, never in the middle or at the end of one.

* lisp/progmodes/cc-engine.el (c-maybe-re-mark-raw-string): Reformulate the
cond form into a `when' form, with all but the first arm of the cond
discarded.
1 job for master in 7 minutes and 18 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
failed #1862
test-all

00:07:18

 
Name Stage Failure
failed
test-all Test
Makefile:807: recipe for target 'bootstrap-emacs.pdmp' failed
make[2]: *** [bootstrap-emacs.pdmp] Error 255
make[2]: Leaving directory '/builds/emacs/emacs/src'
Makefile:424: recipe for target 'src' failed
make[1]: *** [src] Error 2
make[1]: Leaving directory '/builds/emacs/emacs'
Makefile:1115: recipe for target 'bootstrap' failed
make: *** [bootstrap] Error 2
ERROR: Job failed: exit code 1