lavc: remove libschroedinger encoding and decoding wrappers

The library has stopped being developed and Debian has removed it
from its repositories citing security issues.
The native Dirac decoder supports everything the library has and basic
encoding support is still provided via the native vc2 (Dirac Pro, intra
only version of Dirac) encoder. Hence, there's no reason to still support
linking to the library and potentially leading users into security issues.
