mono: remove dontStrip from derivation

This was introduced in 2010 in 9e16b812 and as far as I can tell the
issue mentioned in the comment does not exist anymore. In a stripped
mono build, I was able to:

- Run "mcs --version" without trouble (original test case)
- Run "keepass" (mono app)
- Build packages that depend on mono (e.g. f# and others).

$ nix path-info -sh
Before: 537.8M
After:  304.3M

$ nix path-info -Sh
Before: 1.2G
After:  867.0M

(Transitive closure size decreases more than the package size itself
because mono ended up depending on gcc through leaked paths in .so files
before.)

pkgs/development/compilers/mono/generic.nix

@@ -42,10 +42,6 @@ stdenv.mkDerivation rec {
     ./autogen.sh --prefix $out $configureFlags
   '';
 
-  # Attempt to fix this error when running "mcs --version":
-  # The file /nix/store/xxx-mono-2.4.2.1/lib/mscorlib.dll is an invalid CIL image
-  dontStrip = true;
-
   # We want pkg-config to take priority over the dlls in the Mono framework and the GAC
   # because we control pkg-config
   patches = [ ./pkgconfig-before-gac.patch ];