[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/PECOFF/ObjectFilePECOFF.cpp

@@ -1208,5 +1208,3 @@ ObjectFile::Strata ObjectFilePECOFF::CalculateStrata() { return eStrataUser; }
 
 // PluginInterface protocol
 ConstString ObjectFilePECOFF::GetPluginName() { return GetPluginNameStatic(); }
-
-uint32_t ObjectFilePECOFF::GetPluginVersion() { return 1; }