Remove stale note about pending QPY support for SymbolicPulse

The support for serializing SymbolicPulse objects using QPY was added in
PR Qiskit#7300 and has been released as part of 0.21.0. However, the
documentation for the SymbolicPulse class had a note saying the QPY
support was pending. This was necessary when the SymbolicPulse class was
added in Qiskit#7821 because it was unclear when we'd be able to add the QPY
support. But, Qiskit#7300 was updated and merged soon after Qiskit#7821, but we
neglected to remove that note. This commit corrects the oversight and
removes the stale note.

qiskit/pulse/library/symbolic_pulses.py

@@ -374,11 +374,6 @@ def Sawtooth(duration, amp, freq, name):
     even within the environment not having original class definition loaded.
     This mechanism also allows us to easily share a pulse program including
     custom pulse instructions with collaborators.
-
-    .. note::
-
-        Currently QPY serialization of :class:`SymbolicPulse` is not available.
-        This feature will be implemented shortly.
     """
 
     __slots__ = (