Revert "[JVM] Avoid StringIndexOutOfBoundsException"

This reverts commit 07b4f74.

This fix is not needed since 0effc37. That other commit
made sure $execname is always an absolute path. With an absolute
path the path manipulation logic here works without modification.

src/main.nqp

@@ -23,9 +23,8 @@ my $config := nqp::backendconfig();
 my $sep := $config<osname> eq 'MSWin32' ?? '\\' !! '/';
 #?if jvm
 my $execname := nqp::atkey(nqp::jvmgetproperties,'perl6.execname');
-my $rindex-pos := nqp::rindex($execname, $sep, nqp::rindex($execname, $sep) - 1);
 my $install-dir := $execname ne ''
-    ?? nqp::substr($execname, 0, nqp::rindex($execname, $sep, $rindex-pos < 0 ?? nqp::chars($execname) !! $rindex-pos))
+    ?? nqp::substr($execname, 0, nqp::rindex($execname, $sep, nqp::rindex($execname, $sep) - 1))
     !! $comp.config<prefix>;
 #?endif
 #?if moar

src/perl6-debug.nqp

@@ -471,9 +471,8 @@ sub MAIN(*@ARGS) {
     my $sep := $config<osname> eq 'MSWin32' ?? '\\' !! '/';
 #?if jvm
     my $execname := nqp::atkey(nqp::jvmgetproperties,'perl6.execname');
-    my $rindex-pos := nqp::rindex($execname, $sep, nqp::rindex($execname, $sep) - 1);
     my $install-dir := $execname ne ''
-        ?? nqp::substr($execname, 0, nqp::rindex($execname, $sep, $rindex-pos < 0 ?? nqp::chars($execname) !! $rindex-pos))
+        ?? nqp::substr($execname, 0, nqp::rindex($execname, $sep, nqp::rindex($execname, $sep) - 1))
         !! $comp.config<prefix>;
 #?endif
 #?if moar