Revert "Add --prune option to fetch when syncing a mirror repo"
Revert "Add --prune option to fetch when syncing a mirror repo"

For some users it is not desirable to remove refs that don't exist
on the remote server when syncing a mirror repo.

This reverts commit b4d43b9f664d6472b6c1e91c98f951037d00cea5.

Change-Id: Ie849b66682138ef88da6cd1a5fbb27e993197dd7

11NJ/git-repo,iAlios/git-repo,FuangCao/repo,biaolv/git-repo,xyyangkun/git-repo,qioixiy/git-repo,couchbasedeps/git-repo,chenzilin/git-repo,GatorQue/git-repo-flow,GatorQue/git-repo-flow,GerritCodeReview/git-repo,wavecomp/git-repo,jingyu/git-repo,kdavis-mozilla/repo-repo,4455jkjh/repo,nuclearmistake/repo,testbetta/repo-pub,GerritCodeReview/git-repo,4455jkjh/repo,lewixliu/git-repo,mer-tools/git-repo,ediTLJ/git-repo,chzhong/git-repo,lipro-yocto/git-repo,mozilla/git-repo,codetutil/git-repo,ediTLJ/git-repo,mixedpuppy/git-repo,wavecomp/git-repo,ronan22/repo,iAlios/git-repo,mozilla/git-repo,CyanogenMod/tools_repo,11NJ/git-repo,zemug/repo,codetutil/git-repo,lipro-yocto/git-repo,aosp-mirror/tools_repo,gbraad/git-repo,ronan22/repo,gabbayo/git-repo,FlymeOS/repo,mixedpuppy/git-repo,testbetta/repo-pub,qioixiy/git-repo,CyanogenMod/tools_repo,dodocat/git-repo,FuangCao/git-repo,aosp-mirror/tools_repo,FlymeOS/repo,FuangCao/git-repo,TheQtCompany/git-repo,frogbywyplay/git-repo,jingyu/git-repo,lewixliu/git-repo,FuangCao/repo,frogbywyplay/git-repo,testbetta/git-repo-pub,mer-tools/git-repo,kdavis-mozilla/repo-repo,simbasailor/git-repo,zemug/repo,chenzilin/git-repo,gabbayo/git-repo,xyyangkun/git-repo,testbetta/git-repo-pub,chzhong/git-repo,TheQtCompany/git-repo,dodocat/git-repo,slfyusufu/repo,biaolv/git-repo,nuclearmistake/repo,couchbasedeps/git-repo,slfyusufu/repo,gbraad/git-repo,simbasailor/git-repo