[libcxx] Remove xfails due to picolibc's support for char16_t and char32_t

picolibc recently
([link](picolibc/picolibc@f6f6875))
added support for char16_t and char32_t.

These xfails aren't needed anymore.

Author: vhscampos

libcxx/test/std/depr/depr.c.headers/uchar_h.compile.pass.cpp

@@ -11,9 +11,6 @@
 // Apple platforms don't provide <uchar.h> yet, so these tests fail.
 // XFAIL: target={{.+}}-apple-{{.+}}
 
-// mbrtoc16 not defined.
-// XFAIL: LIBCXX-PICOLIBC-FIXME
-
 // <uchar.h>
 
 #include <uchar.h>

libcxx/test/std/strings/c.strings/cuchar.compile.pass.cpp

@@ -11,9 +11,6 @@
 // Apple platforms don't provide <uchar.h> yet, so these tests fail.
 // XFAIL: target={{.+}}-apple-{{.+}}
 
-// mbrtoc16 not defined.
-// XFAIL: LIBCXX-PICOLIBC-FIXME
-
 // <cuchar>
 
 #include <cuchar>