[lldb] Remove unimplemented StackFrame::BehavesLikeZerothFrame

Commit [1] added a declaration of function-member
StackFrame::BehavesLikeZerothFrame but hasn't added an implementation
for the function. This commit removes this declation, because the
function is not used anywhere.

[1] 31e6dbe1 Fix PC adjustment in StackFrame::GetSymbolContext

Differential Revision: https://reviews.llvm.org/D75979

Patch by Anton Kolesov <Anton.Kolesov@synopsys.com>