texture-cache: remove unused base64 code path

This was introduced in b0c6cf3f to support image data for web search
providers, which do not exist anymore. Just remove it.

https://bugzilla.gnome.org/show_bug.cgi?id=736936
background: port to new GFile MetaBackground API

https://bugzilla.gnome.org/show_bug.cgi?id=736936
st: always use GFile internally

We're moving the theme infrastructure towards GResource, so as a first
step move all the loading to use GFiles instead of URIs or paths.

https://bugzilla.gnome.org/show_bug.cgi?id=736936
theme: convert stylesheet loading to GFile

In preparation to making it a GResource.

https://bugzilla.gnome.org/show_bug.cgi?id=736936
theme: make a GResource

Now that we have all the infrastructure ready, port the theme to a
GResource.

https://bugzilla.gnome.org/show_bug.cgi?id=736936