remove obsolete link_directories command
remove obsolete link_directories command

It's not necessary, as the target_link_libraries command contains an
absolute path already, and the path given doesn't exist anymore,
leading only to linker warnings like:
ld: warning: directory not found for option
'-L/Users/travis/build/google/googletest/build/googlemock/gtest/src'

jlanecox/googletest,google/googletest,jlanecox/googletest,gianricardo/googletest,google/googletest,xnagireddy/googletest,gianricardo/googletest,fuchsia-mirror/third_party-googletest,google/googletest,empiredan/googletest,antidotcb/googletest,empiredan/googletest,antidotcb/googletest,antidotcb/googletest,google/googletest,fuchsia-mirror/third_party-googletest,jlanecox/googletest,fuchsia-mirror/third_party-googletest,xnagireddy/googletest,gianricardo/googletest,jlanecox/googletest,fuchsia-mirror/third_party-googletest,empiredan/googletest,xnagireddy/googletest,xnagireddy/googletest,antidotcb/googletest