build: remove duplicit warning suppression

These warnings have already been added to $dontwarn.

m4/virt-compile-warnings.m4

@@ -119,9 +119,6 @@ AC_DEFUN([LIBVIRT_COMPILE_WARNINGS],[
     # ideally we'd turn many of them on
     dontwarn="$dontwarn -Wfloat-equal"
     dontwarn="$dontwarn -Wdeclaration-after-statement"
-    dontwarn="$dontwarn -Wcast-qual"
-    dontwarn="$dontwarn -Wconversion"
-    dontwarn="$dontwarn -Wsign-conversion"
     dontwarn="$dontwarn -Wpacked"
     dontwarn="$dontwarn -Wunused-macros"
     dontwarn="$dontwarn -Woverlength-strings"