CMP0054: Remove ALLOWED_UNKNOWN_VARIABLE_READ_ACCESS

The CMake code for it is now redundant and only slows down uninitialized
variable accesses.
tests: test CMP0054
Merge topic 'dev/watch-variable-policy' into next

6a6f2b63 tests: test CMP0054
c0e4340a CMP0054: Remove ALLOWED_UNKNOWN_VARIABLE_READ_ACCESS