configure: Remove unsupported -Wunsigned-char

This warning option is unsupported and I was unable to find any
reference that it was ever supported, so remove it.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>

configure.ac

@@ -252,7 +252,6 @@ WARNLIST="
 	format-security
 	format-nonliteral
 	no-long-long
-	unsigned-char
 	no-strict-aliasing
 	"