[lldb][DWARF] Remove m_forward_decl_die_to_compiler_type as it never actually being used. (#89427)

This removes `m_forward_decl_die_to_compiler_type` which is a map from
`const DWARFDebugInfoEntry *` to `lldb::opaque_compiler_type_t`. This
map is currently used in `DWARFASTParserClang::ParseEnum` and
`DWARFASTParserClang::ParseStructureLikeDIE` to avoid creating duplicate
CompilerType for the specific DIE. But before entering these two
functions in `DWARFASTParserClang::ParseTypeFromDWARF`, we already
checked with `SymbolFileDWARF::GetDIEToType()` if we have a Type created
from this DIE to avoid trying to parse the same DIE twice. So, this map
is unnecessary and not useful.