clean_up: Rename reference to thread

The name thread gives the impression this is
a multi-threaded system which is not the case.
This patch removes all references to thread in the codebase.

Signed-off-by: Tarek El-Sherbiny <tarek.el-sherbiny@arm.com>
Change-Id: Ibcef650c2d0ffd40f8318819918663fbbb843df0