libbinder_ndk: remove global lock on proxy delete

We don't need a global lock here. Actually, it can be removed completely
(though that will have to be in a follow-up, it might require extra
allocations in some cases).

Bug: 192023359
Test: CtsNdkBinderTestCases
Change-Id: If5ad5acf003832fa69fe9c985a060feb53bea7af