Remove etcprovider plugin.

Remove etcprovider plugin.

Handling image providers via QDeclarativeExtensionPlugin is not a
pattern we want to promote for library code. The plugin was also
missing documentation and autotests. It might be appropriate to be
re-added later as an example showing the use of an image provider with
a texture factory.

Change-Id: I2d8f3208aebd55180bd627b3f9acc0679eb6328c
Reviewed-by: Martin Jones <martin.jones@nokia.com>