refactor: delete build option 'enable_vhe'

These set of changes are making VHE enabled by default. This means, the
'enable_vhe' configuration flag is not needed in the Hafnium build
targets.

Signed-off-by: J-Alves <joao.alves@arm.com>
Change-Id: I35365ea932587d75e3a3ef913adc04b13db23001