[gdb/symtab] Fix data race in ~charset_vector

[gdb/symtab] Fix data race in ~charset_vector

When doing:
...
$ gdb ./outputs/gdb.ada/char_enum_unicode/foo -batch -ex "break foo.adb:26"
...
with a gdb build with -fsanitize=thread I run into a data race:
...
WARNING: ThreadSanitizer: data race (pid=30917)
  Write of size 8 at 0x7b0400004070 by main thread:
    #0 free <null> (libtsan.so.2+0x4c5e2)
    #1 xfree<char> gdbsupport/gdb-xfree.h:37 (gdb+0x650f17)
    #2 charset_vector::clear() gdb/charset.c:703 (gdb+0x651354)
    #3 charset_vector::~charset_vector() gdb/charset.c:697 (gdb+0x6512d3)
    #4 <null> <null> (libtsan.so.2+0x32643)
    #5 captured_main_1 gdb/main.c:1310 (gdb+0xa3975a)
...

The problem is that we're freeing the charset_vector elements in the destructor,
which may still be used by a worker thread.

Fix this by not freeing the charset_vector elements in the destructor.

Tested on x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29311