Remove mixed-source.patch from zlib.

With SPDY gone, this is no longer necessary. This change was done by
reversing simd.patch, reversing mixed-source.patch, reapplying
simd.patch, and then recreating simd.patch.

BUG=none

Review-Url: https://codereview.chromium.org/2669053004
Cr-Original-Commit-Position: refs/heads/master@{#447853}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: f2095b9da969dd7decf3224fc96ffe8ef5f5455b