Read-only COWs should not be exempt from s/// croaking

Read-only COWs should not be exempt from s/// croaking

$  ./miniperl -Ilib -e 'for(__PACKAGE__) { s/a/a/ }'
Modification of a read-only value attempted at -e line 1.
$  ./miniperl -Ilib -e 'for(__PACKAGE__) { s/b/b/ }'
$  ./miniperl -Ilib -e 'for("main") { s/a/a/ }'
Modification of a read-only value attempted at -e line 1.
$  ./miniperl -Ilib -e 'for("main") { s/b/b/ }'
Modification of a read-only value attempted at -e line 1.

When I pass the constant "main" to s///, it croaks whether the regular
expression matches or not.

When I pass __PACKAGE__, which has the same content and is also read-
only, it only croaks when the pattern matches.

This commit removes some logic that is left over from when
READONLY+FAKE meant copy-on-write.  Read-only does mean read-only now,
so copy-on-write scalars should not be exempt from read-only checks.