gdb/ctf.c: Get rid of mkdir redefinition

Making GDB use gnulib's C++ namespace support shows this build error
on mingw:

 ../../src/gdb/ctf.c: In function 'void ctf_start(trace_file_writer*, const char*)':
 ../../src/gdb/ctf.c:309:46: error: no match for call to '(const gnulib::_gl_mkdir_wrapper) (const char*&)'
  #define mkdir(pathname, mode) mkdir (pathname)
					       ^
 ../../src/gdb/ctf.c:327:15: note: in expansion of macro 'mkdir'
    if (gnulib::mkdir (dirname, hmode) && errno != EEXIST)
		^
 ../../src/gdb/ctf.c:309:46: note: candidate: gnulib::_gl_mkdir_wrapper::type {aka int (*)(const char*, short unsigned int)} <conversion>
  #define mkdir(pathname, mode) mkdir (pathname)
					       ^
 ../../src/gdb/ctf.c:327:15: note: in expansion of macro 'mkdir'
    if (gnulib::mkdir (dirname, hmode) && errno != EEXIST)
		^
 ../../src/gdb/ctf.c:309:46: note:   candidate expects 3 arguments, 2 provided
  #define mkdir(pathname, mode) mkdir (pathname)
					       ^
 ../../src/gdb/ctf.c:327:15: note: in expansion of macro 'mkdir'
    if (gnulib::mkdir (dirname, hmode) && errno != EEXIST)
		^

The problem is the mkdir define.  We can just remove it, since
gnulib's replacement already takes care of this prototype difference
detail.