[lldb] Remove __FUNCTION__ from log messages in lldbHost (NFC)

LLDB's logging infrastructure supports prepending log messages with the
name of the file and function that generates the log (see help log
enable). Therefore it's unnecessary to include the current __FUNCTION__
in the log message itself. This patch removes __FUNCTION__ from log
messages in the Host library.

Differential revision: https://reviews.llvm.org/D151762