Add explanation for workaround in build script

This also removes the unused MONO_OPTIONS export.

Author: bording

build.libgit2sharp.sh

@@ -2,8 +2,10 @@
 
 EXTRADEFINE="$1"
 
-export MONO_OPTIONS=--debug
-export LD_LIBRARY_PATH=$TRAVIS_BUILD_DIR/LibGit2Sharp/bin/Release:$LD_LIBRARY_PATH
+# Exporting a dummy value with ':' works around a linux mono bug where it
+# can't find libgit when the libgit2sharp assembly has been shadow copied.
+# This appears to only be a problem on linux and not osx.
+export LD_LIBRARY_PATH=:
 
 # Required for NuGet package restore to run.
 mozroots --import --sync