Revert inclusion of sphinx.ext.imgconverter

This was added in #2820 to convert GIFs in the PDF output,
but it also converts them in HTML...

Fixes #2861.

conf.py

@@ -11,7 +11,7 @@ needs_sphinx = '1.3'
 
 # Sphinx extension module names and templates location
 sys.path.append(os.path.abspath('extensions'))
-extensions = ['gdscript', 'sphinx_tabs.tabs', 'sphinx.ext.imgmath', 'sphinx.ext.imgconverter']
+extensions = ['gdscript', 'sphinx_tabs.tabs', 'sphinx.ext.imgmath']
 
 templates_path = ['_templates']