Revert "Make Match.STR faster if there are no captures"

This reverts commit 495ddcc.

This was only an optimization in an area that could use a lot of
thinking and reorganization.  Since it apparently causes issues:

    https://colabti.org/irclogger/irclogger_log/moarvm?date=2020-02-21#l112

revert it.

src/core.c/Match.pm6

@@ -12,14 +12,11 @@ my class Match is Capture is Cool does NQPMatchRole {
     method Int(--> Int:D) { self.Str.Int }
 
     method STR() {
-        nqp::getattr(self,Match,'$!regexsub').CAPS.has-captures
-          ?? self!MATCH.Str
-          !! nqp::substr(
-               self.orig,
-               nqp::getattr_i(self,Match,'$!from'),
-               nqp::getattr_i(self,Match,'$!pos')
-                 - nqp::getattr_i(self,Match,'$!from')
-             )
+        nqp::if(
+          nqp::istype(nqp::getattr(self,Match,'$!match'), NQPdidMATCH),
+          self.Str,
+          self!MATCH.Str
+        )
     }
 
     method MATCH() {