[libc++][chrono] Removes format include.

Switch to the new granular format_functions header. Since the chrono's
format dependency in C++20 hasn't been in a release it's save to remove
it.

Depends on D133665

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D133796

libcxx/include/__chrono/ostream.h

@@ -13,7 +13,7 @@
 #include <__chrono/day.h>
 #include <__chrono/statically_widen.h>
 #include <__config>
-#include <format>
+#include <__format/format_functions.h>
 #include <ostream>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)

libcxx/test/libcxx/transitive_includes/cxx20/expected.chrono

@@ -20,7 +20,6 @@ ctime
 cwchar
 cwctype
 exception
-format
 functional
 initializer_list
 ios

libcxx/test/libcxx/transitive_includes/cxx2b/expected.chrono

@@ -18,7 +18,6 @@ ctime
 cwchar
 cwctype
 exception
-format
 initializer_list
 ios
 iosfwd