lavf: Remove a now useless parameter to ffurl_register_protocol

This was added in 9b07a2dc02e9 as an ABI hack to allow older
code built with lavf 52 to register protocols even if the size
of the URLProtocol struct was increased. Later, registering
protocols from outside of lavf was removed and this workaround
isn't needed any longer since lavf 53.

This removes an unchecked malloc and a memory leak for the cases
when this workaround actually was used - which it hasn't since
lavf 53.

Signed-off-by: Martin Storsjö <martin@martin.st>