gdb/dwarf2/read.c: remove pessimizing std::move

gdb/dwarf2/read.c: remove pessimizing std::move

When building with this clang:

    $ c++ --version
    FreeBSD clang version 16.0.6 (https://github.com/llvm/llvm-project.git llvmorg-16.0.6-0-g7cbf1a259152)

I see:

    $ gmake
      CXX    dwarf2/read.o
    /home/smarchi/src/binutils-gdb/gdb/dwarf2/read.c:4890:6: error: moving a temporary object prevents copy elision [-Werror,-Wpessimizing-move]
                                            std::move (thread_storage.release_parent_map ()));
                                            ^
    /home/smarchi/src/binutils-gdb/gdb/dwarf2/read.c:4890:6: note: remove std::move call here
                                            std::move (thread_storage.release_parent_map ()));
                                            ^~~~~~~~~~~                                    ~

The compiler seems right, there is not need to std::move the result of
`release_parent_map ()`, it's already going to be an rvalue.  Remove the
std::move.

The issue isn't FreeBSD-specific, I see it on Linux as well when
building hwith clang, I just noticed it on a FreeBSD build first.

Change-Id: I7aa20a4db56c799f20d838ad08099a01653bba19
Approved-By: Tom Tromey <tom@tromey.com>