Don't clear g_disc_raw_data_buf for start of LE service discovery

It's used just for SDP, and is cleared inside bta_dm_find_services
before it starts. This is not necesary.

Bug: 330675788
Bug: 335732980
Flag: com.android.bluetooth.flags.separate_service_and_device_discovery
Test: mma -j32
Change-Id: I30cee32873169d7154b0573574f31e6b6f3a7e39