Simplify internal QX sub

This is the workhorse behind qx/ /, but since this will never get
called with the :cwd or :env arguments, handling them here seems
unnecessary.  So don't.

This sub should probably become a class method of Proc, and codegenning
adapted accordingly, as to avoid poisoning of the core setting.

src/core.c/Proc.rakumod

@@ -242,9 +242,9 @@ multi sub shell($cmd, :$in = '-', :$out = '-', :$err = '-',
     $proc
 }
 
-sub QX($cmd, :$cwd = $*CWD, :$env) is implementation-detail {
+sub QX($cmd) is implementation-detail {
     my $proc := Proc.new(:out);
-    $proc.shell($cmd, :$cwd, :$env);
+    $proc.shell($cmd);
     $proc.out.slurp(:close) // "Unable to read from '$cmd'".Failure
 }