Remove deprecated ParseFlag/UnparseFlag declaration for absl::Time.

To match earlier removal of the definition of these functions

PiperOrigin-RevId: 932983236
Change-Id: I5cbd97a0175027822e854bb6a3f5553f4c993963

absl/time/time.h

diff --git a/absl/time/time.h b/absl/time/time.h
index 03cf67a..fc1456b 100644
--- a/absl/time/time.h
+++ b/absl/time/time.h
@@ -1064,11 +1064,6 @@
 // the format specified by `absl::ParseTime()`.
 std::string AbslUnparseFlag(Time t);
 
-ABSL_DEPRECATED("Use AbslParseFlag() instead.")
-bool ParseFlag(const std::string& text, Time* t, std::string* error);
-ABSL_DEPRECATED("Use AbslUnparseFlag() instead.")
-std::string UnparseFlag(Time t);
-
 // TimeZone
 //
 // The `absl::TimeZone` is an opaque, small, value-type class representing a