resolv: delete unused RES_F_* flags

 - RES_F_CONN was write-only (cleared, but never tested)
 - RES_F_LASTMASK / RES_F_LASTSHIFT were write-only
 - RES_GETLAST(flags) was never used
 - res_findzonecut2() doesn't exist

Test: atest resolv_integration_test
Change-Id: I513c039e3ce48fd8a13ee4d1aad3d6d051c3e856