Fix calculating the size of wxSpinCtrl numeric box under wxOSX

Since 8189ce89ed (Improve wxTextCtrl::DoGetSizeFromTextSize() in wxOSX, 2020-05-28) GetSizeFromTextSize() is fully functional under wxOSX
so we can revert 39d586421b (Don't call GetSizeFromText() under wxOSX, 2020-05-21) and use GetSizeFromText() in calculating text box size.

Closes #18766.

src/generic/spinctlg.cpp

@@ -117,9 +117,6 @@ public:
         event.Skip();
     }
 
-#if defined(__WXMSW__) || defined(__WXGTK__)
-    // GetSizeFromTextSize() is not implemented in wxOSX
-    // so GetSizeFromText() cannot be used for size calculations.
     virtual wxSize DoGetBestSize() const wxOVERRIDE
     {
         wxString minVal = m_spin->DoValueToText(m_spin->m_min);
@@ -129,7 +126,6 @@ public:
 
         return wxSize(wxMax(minValSize.x, maxValSize.x), wxMax(minValSize.y, maxValSize.y));
     }
-#endif // __WXMSW || __WXGTK__
 
     wxSpinCtrlGenericBase *m_spin;