disable_mmu() cannot work as a C function as there is no control
over data accesses generated by the compiler between disabling and
cleaning the data cache. This results in reading stale data from
main memory.

This patch removes the C version of this function in juno code.
An assembly version has been introduced in commit 2f5dcfef1db4.

Change-Id: I0de10dbe2db8d22855bf1f60f1e48540a4861cb6