Remove obsolete optimization from ThemeManager

Since the ThemeManager provided by ThemeManagerHelper is shared between
most tests, overriding the ThemeManager's default resource loader with
a buffering implementation does not affect the test suite's performance
anymore.

Change-Id: If141065bf7384fef35b262ec10de96152c094763
Signed-off-by: Ralf Sternberg <rsternberg@eclipsesource.com>