Removed unused constants

Ignore-this: 2ed954b9f5f352f6b1feae3acc3382bb

darcs-hash:20130418111326-56b4d-878ed59f2f45baa931ecbf1feedd079e5cb6f4e2
Fixed memory leak

Ignore-this: dfa64777f2093e4fa63be5c2ce6fbe3f

It was introduced long ago to avoid crashes, get rid of it
now that the core uses a new PJSIP version with native audio backends.

darcs-hash:20130422105113-56b4d-be3c0f17dd16853b9b0e02b0df50417793a54b41
Removed engine shutdown workaround

Ignore-this: b1d38f31f526d5eecfb7a76295deb7a3

darcs-hash:20130423082110-56b4d-e880e01804d75bd70a9917de83525cac949d2a7d