[lldb] Remove PluginInterface::GetPluginVersion

In all these years, we haven't found a use for this function (it has
zero callers). Lets just remove the boilerplate.

Differential Revision: https://reviews.llvm.org/D109600

lldb/source/Plugins/ObjectFile/Mach-O/ObjectFileMachO.cpp

@@ -6173,8 +6173,6 @@ lldb_private::ConstString ObjectFileMachO::GetPluginName() {
   return GetPluginNameStatic();
 }
 
-uint32_t ObjectFileMachO::GetPluginVersion() { return 1; }
-
 Section *ObjectFileMachO::GetMachHeaderSection() {
   // Find the first address of the mach header which is the first non-zero file
   // sized section whose file offset is zero. This is the base file address of