bta_dm: Remove canceling service discovery on BTA_DmSearchCancel()

This patch removes canceling of ongoing service discovery.
It was done on BTA_DmSearchCancel() which should cancel BTA_DmSearch()
operations.
Service discovery is started with BTA_DmDiscover() and seems that
it should not be canceled unless link is disconnected.

Main problem which is fixed by this patch is following scenario

1. Device A is bonded and service discovery is started
2. Device B create bond has started and user accepted it
3. After 2), BTA_DmSearchCancel() is called to stop Inquiry and LE scan,
   however this function also stops Service Discovery started by
BTA_DmDiscover()
4. Because of 3), Device A is bonded but without any known services and
   there is no way to trigger service discovery again unless unbond and
   bonding again.

Issue can be easily reproduced with 2 Hearing Aids if bonding
has been called not from AOSP Settings but from external application which
can do it fast.
This issue might also create problems with future LeAudio devices.

test: atest -b -i -t -c BluetoothInstrumentationTests
sponsor: jpawlowski@

Reviewed-on: https://gerrit.codecoup.pl/c/aosp/platform/system/bt/+/3616


Reviewed-by: Jakub Tyszkowski <jakub.tyszkowski@codecoup.pl>
Change-Id: Ie944b705d908d4b9283b99da0e36854b0b72d09e