Stop ignoring the ubuntu precise sysroot.

We added a new sysroot in r414564 and reverted the change in
r414594, but we left the .gitignore entries in. I suspect
that this is keeping the sysroot directory from being cleaned
up, and the license checks are still failing.

TBR=thomasanderson@chromium.org, jianli@chromium.org, haraken@chromium.org
NOTRY=true
BUG=638394

Review-Url: https://codereview.chromium.org/2279143002
Cr-Commit-Position: refs/heads/master@{#414646}

.gitignore

@@ -74,7 +74,6 @@ vs-chromium-project.txt
 /build/x64/
 /build/linux/bin/eu-strip
 /build/linux/debian_*-sysroot/
-/build/linux/ubuntu_*-sysroot/
 /build/mac_files
 /buildtools
 # The Chrome OS build creates a /c symlink due to http://crbug.com/54866.