configure: remove test for zlib.h
configure: remove test for zlib.h

hasn't been needed since:
  9523f2a Add Alpha Encode support from WebPEncode.

Change-Id: Ia784ac434ec6fd43aade8875a91e2ad9f0fa9c30

Tekserve/libwebp,Maria1099/webm.libwebp,rxl194/libwebp,gshORTON/webm.libwebp,Acidburn0zzz/webm.libwebp,imazen/libwebp,redengineer/libwebp,PKRoma/libwebp,Distrotech/libwebp,webmproject/libwebp,gshORTON/webm.libwebp,ttyangf/libwebp,Suvarna1488/webm.libwebp,ttyangf/libwebp,imazen/libwebp,matanbs/webm.libwebp,ya7lelkom/libwebp,reimaginemedia/webm.libwebp,rxl194/libwebp,comoyo/gonzo-libwebp,jiehu5114/libwebp,comoyo/gonzo-libwebp,zofuthan/libwebp,jiehu5114/libwebp,keane83/BLE_IM,reimaginemedia/webm.libwebp,zofuthan/libwebp,kleopatra999/webm.libwebp,cysp/libwebp,ya7lelkom/libwebp,Aexyn/libwebp,redengineer/libwebp,gshORTON/webm.libwebp,jacklicn/libwebp,Suvarna1488/webm.libwebp,altogother/webm.libwebp,reimaginemedia/webm.libwebp,altogother/webm.libwebp,Distrotech/libwebp,Suvarna1488/webm.libwebp,imazen/libwebp,comoyo/gonzo-libwebp,Maria1099/webm.libwebp,iniwf/webm.libwebp,Maria1099/webm.libwebp,Acidburn0zzz/webm.libwebp,iniwf/webm.libwebp,dumganhar/libwebp,ttyangf/libwebp,Aexyn/libwebp,altogother/webm.libwebp,webmproject/libwebp,abwiz0086/webm.libwebp,matanbs/webm.libwebp,comoyo/gonzo-libwebp,redengineer/libwebp,keane83/BLE_IM,rxl194/libwebp,Tekserve/libwebp,kim42083/webm.libwebp,kalli123/webm.libwebp,zofuthan/libwebp,dumganhar/libwebp,appunite/libwebp,Distrotech/libwebp,keane83/BLE_IM,ericmckean/webm.libwebp,altogother/webm.libwebp,keane83/BLE_IM,kim42083/webm.libwebp,jacklicn/libwebp,Acidburn0zzz/webm.libwebp,webmproject/libwebp,Aexyn/libwebp,rxl194/libwebp,iniwf/webm.libwebp,keane83/BLE_IM,appunite/libwebp,webmproject/libwebp,cysp/libwebp,rxl194/libwebp,Tekserve/libwebp,PKRoma/libwebp,kim42083/webm.libwebp,kleopatra999/webm.libwebp,ericmckean/webm.libwebp,cysp/libwebp,comoyo/gonzo-libwebp,ericmckean/webm.libwebp,yohunl/libwebp,PKRoma/libwebp,cysp/libwebp,Maria1099/webm.libwebp,ya7lelkom/libwebp,redengineer/libwebp,reimaginemedia/webm.libwebp,ttyangf/libwebp,redengineer/libwebp,kim42083/webm.libwebp,imazen/libwebp,comoyo/gonzo-libwebp,gshORTON/webm.libwebp,imazen/libwebp,Aexyn/libwebp,kleopatra999/webm.libwebp,jiehu5114/libwebp,ya7lelkom/libwebp,kalli123/webm.libwebp,kleopatra999/webm.libwebp,Acidburn0zzz/webm.libwebp,jiehu5114/libwebp,PKRoma/libwebp,appunite/libwebp,ya7lelkom/libwebp,PKRoma/libwebp,altogother/webm.libwebp,PKRoma/libwebp,yohunl/libwebp,jiehu5114/libwebp,appunite/libwebp,yohunl/libwebp,webmproject/libwebp,kalli123/webm.libwebp,yohunl/libwebp,dumganhar/libwebp,yohunl/libwebp,appunite/libwebp,zofuthan/libwebp,abwiz0086/webm.libwebp,jacklicn/libwebp,abwiz0086/webm.libwebp,matanbs/webm.libwebp,Tekserve/libwebp,matanbs/webm.libwebp,kalli123/webm.libwebp,Maria1099/webm.libwebp,ericmckean/webm.libwebp,gshORTON/webm.libwebp,reimaginemedia/webm.libwebp,kleopatra999/webm.libwebp,Suvarna1488/webm.libwebp,comoyo/gonzo-libwebp,jacklicn/libwebp,jacklicn/libwebp,appunite/libwebp,iniwf/webm.libwebp,kalli123/webm.libwebp,ttyangf/libwebp,zofuthan/libwebp,Aexyn/libwebp,abwiz0086/webm.libwebp,Suvarna1488/webm.libwebp,matanbs/webm.libwebp,iniwf/webm.libwebp,kim42083/webm.libwebp,Acidburn0zzz/webm.libwebp,abwiz0086/webm.libwebp,ericmckean/webm.libwebp