SL-18837: Clean up more set-but-unused variables for Xcode 14.3

indra/llui/llbutton.cpp

indra/llui/llmenugl.cpp

indra/newview/llagent.cpp

indra/newview/llmodelpreview.cpp

indra/newview/llpanelnearbymedia.cpp

indra/newview/llsceneview.cpp

indra/newview/llselectmgr.cpp

indra/newview/llsprite.cpp

indra/newview/llstartup.cpp

indra/newview/lltextureview.cpp

indra/newview/llviewerobjectlist.cpp

indra/newview/llvosurfacepatch.cpp

indra/newview/llvovolume.cpp