remove second declaration of MVM_frame_destroy

seems like msvc (at least the one used on appveyor) didn't
like this. Perhaps it was a left-over from typo-ing destroy
as destory a few lines below.

Author: timo

src/core/frame.h

@@ -178,7 +178,6 @@ struct MVMInvocationSpec {
 
 MVMRegister * MVM_frame_initial_work(MVMThreadContext *tc, MVMuint16 *local_types,
                                      MVMuint16 num_locals);
-void MVM_frame_destroy(MVMThreadContext *tc, MVMFrame *frame);
 void MVM_frame_invoke_code(MVMThreadContext *tc, MVMCode *code,
                            MVMCallsite *callsite, MVMint32 spesh_cand);
 void MVM_frame_invoke(MVMThreadContext *tc, MVMStaticFrame *static_frame,