Remove ts_allocator from api.h
Remove ts_allocator from api.h

Since we have exposed the function to set allocator, we don't need to
expose the global variable in `api.h`.

tree-sitter/tree-sitter,tree-sitter/tree-sitter,tree-sitter/tree-sitter,tree-sitter/tree-sitter,tree-sitter/tree-sitter,tree-sitter/tree-sitter,tree-sitter/tree-sitter