buildMozillaMach: stop deleting the configure script

They're not autogenerated files any longer, and have been shipped as part
of the source tree since roughly Aug 2022.

Related: mozbz#1787977 ("Include configure in the tree")
Closes: #306937

pkgs/applications/networking/browsers/firefox/common.nix

@@ -298,9 +298,6 @@ buildStdenv.mkDerivation {
   setOutputFlags = false; # `./mach configure` doesn't understand `--*dir=` flags.
 
   preConfigure = ''
-    # remove distributed configuration files
-    rm -f configure js/src/configure .mozconfig*
-
     # Runs autoconf through ./mach configure in configurePhase
     configureScript="$(realpath ./mach) configure"