src/svgtiny_css.c: don't check "key" in the libdom user_data handler

src/svgtiny_css.c: don't check "key" in the libdom user_data handler

The libdom user data get/set functions in the libcss select handler
API identify the libcss user data (among all potential user data) with
a key -- basically, a string. In theory, we should look for this key
in the callback function before delegating to libcss (to make sure
it's libcss user data we're working with), but at the moment, libcss
is the only place the data could have come from.

We can therefore skip the key comparison entirely. This simplifies the
code a bit because it's currently awkward to access the same interned
key string from all three sites.