repo: Remove homebrew python installation in CI

The new Travis CI image seems to come with python 3 already installed.
As a result, trying it install it causes a homebrew error.

Change-Id: If627249e3d7ddc8b35df6d31c906cec65b931fec
diff --git a/.travis.yml b/.travis.yml
index ee690a2..7cd6035 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -51,11 +51,6 @@
       sudo apt-get -y install libxkbcommon-dev libwayland-dev libxrandr-dev libx11-xcb-dev
     fi
   - |
-    if [[ "$VULKAN_BUILD_TARGET" == "MACOS" ]]; then
-      # We need to upgrade to a newer python
-      brew upgrade python3
-    fi
-  - |
     if [[ "$VULKAN_BUILD_TARGET" == "LINUX" ]] || [[ "$VULKAN_BUILD_TARGET" == "MACOS" ]]; then
       # Install dependencies
       python scripts/update_deps.py --dir=external $UPDATE_DEPS_EXTRA_OPTIONS