MAP: Remove redundant call to sendShutdownMessage to prevent crash

Usecase:
Perform BT off/on repeatedly.

Issue:
Bluetooth crashes.

Fix:
Remove redundant call to sendShutdownMessage() from stop API to prevent
MapServiceMessageHandler becoming null and being dereferenced.

Bug: 29149190
Change-Id: I739664975043c4a1895fd2113869c87cdb704338
(cherry picked from commit 5847ca3c)