Remove unnecessary zlib warning flags (#1243)

These were used to suppress warnings in zlib 1.2, but now that we are on
zlib 1.3 they are no longer necessary. In fact, they cause warnings with
some compilers so it's better to remove them.

Author: tobil4sk

src/hx/libs/zlib/Build.xml

@@ -13,10 +13,6 @@
   <compilerflag value="-DSTDC" unless="windows" />
   <compilerflag value="-DHAVE_UNISTD_H" unless="windows" />
 
-  <compilerflag value="-Wno-unknown-warning" unless="MSVC_VER" />
-  <compilerflag value="-Wno-unknown-warning-option" unless="MSVC_VER" />
-  <compilerflag value="-Wno-deprecated-non-prototype" unless="MSVC_VER" />
-
   <file name="ZLib.cpp"/>
 
   <!-- HXCPP_LINK_NO_ZLIB may be set too late, so use filterout as well. -->