Don't add a duplicate copy of libtightdb-ios.a to the framework

I'm not entirely clear on how, but libRealm-fat.a already has a copy of
tightdb, so adding a second copy with libtool is unnecessary and makes the
final framework 25% larger.
Merge pull request #723 from realm/tg-duplicate-object-files

Don't add a duplicate copy of libtightdb-ios.a to the framework