Fix leak due to assigning a xstrdup-ed string to the std::string gdb_datadir

Fix leak due to assigning a xstrdup-ed string to the std::string gdb_datadir

Valgrind reports the following leak:
==32623== 56 bytes in 1 blocks are definitely lost in loss record 1,099 of 6,654
==32623==    at 0x4835753: malloc (vg_replace_malloc.c:307)
==32623==    by 0x25CF67: xmalloc (alloc.c:60)
==32623==    by 0x65FBD9: xstrdup (xstrdup.c:34)
==32623==    by 0x413D9E: captured_main_1(captured_main_args*) (main.c:553)
==32623==    by 0x414FFA: captured_main (main.c:1172)
==32623==    by 0x414FFA: gdb_main(captured_main_args*) (main.c:1197)
==32623==    by 0x22531A: main (gdb.c:32)

Commit f2aec7f6d14 changed gdb_datadir to std::string.
So, xstrdup-ing the result of relocate_gdb_directory (returning a std::string)
is not needed and creates a leak.

Fix the leak by removing the xstrdup and the not needed c_str ().
Also removes a useless conversion of gdb_datadir to std::string.

gdb/ChangeLog
2019-10-01  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* main.c (relocate_gdbinit_path_maybe_in_datadir): Remove std::string
	conversion of gdb_datadir.
	(captured_main_1): Remove xstrdup when assigning to gdb_datadir,
	remove not needed c_str ().