sysprop: Remove the wrapper GET_SYSPROP

Originally added to let FLOSS have a different API.
Ia8b81aa0800cc02f82bed1666dd14ffbbbe82bbf removes
this workaround.

Bug: 331817295
Test: m com.android.btservices
Test: ./floss/build/build-in-container.py
Flag: EXEMPT, mechanical refactor
Change-Id: I8d6bdd7cac1da0a1bc76ca6ede4fcd98357cdb0e