Remove PYBIND11_UNSHARED_STATIC_LOCALS macro

The macro isn't doing anything useful now that hidden visibility is
applied to all pybind11 code.

include/pybind11/cast.h

@@ -62,7 +62,7 @@ struct type_info {
     bool module_local : 1;
 };
 
-PYBIND11_UNSHARED_STATIC_LOCALS PYBIND11_NOINLINE inline internals *&get_internals_ptr() {
+PYBIND11_NOINLINE inline internals *&get_internals_ptr() {
     static internals *internals_ptr = nullptr;
     return internals_ptr;
 }
@@ -129,7 +129,7 @@ PYBIND11_NOINLINE inline internals &get_internals() {
 }
 
 // Works like internals.registered_types_cpp, but for module-local registered types:
-PYBIND11_NOINLINE PYBIND11_UNSHARED_STATIC_LOCALS inline type_map<void *> &registered_local_types_cpp() {
+PYBIND11_NOINLINE inline type_map<void *> &registered_local_types_cpp() {
     static type_map<void *> locals{};
     return locals;
 }

include/pybind11/detail/common.h

@@ -79,16 +79,6 @@
 #  endif
 #endif
 
-// Attribute macro for a function containing one or more static local variables that mustn't share
-// the variable across shared objects (for example, because the value might be incompatible for
-// modules compiled under different pybind versions).  This is required under g++ (depending on the
-// specific compiler and linker options), and won't hurt under gcc-compatible compilers:
-#if defined(__GNUG__)
-#  define PYBIND11_UNSHARED_STATIC_LOCALS __attribute__ ((visibility("hidden")))
-#else
-#  define PYBIND11_UNSHARED_STATIC_LOCALS
-#endif
-
 #if defined(_MSC_VER)
 #  define PYBIND11_NOINLINE __declspec(noinline)
 #else