cmake.bbclass: do not search host paths for find_program()

cmake.bbclass: do not search host paths for find_program()

OECMAKE_FIND_ROOT_PATH_MODE_PROGRAM[1] controls the search
behavior of find_program(). When this variable's value was
first used in OE, it was deliberately set to BOTH to allow
searching of host tools. This is to ensure the necessary tools
from host could be used. The downside is that the configure
behavior may differ on different host environments.

Later, this cmake.bbclass was added the ability to search tools
under the HOSTTOOLS_DIR. This means we no longer needs cmake to
search the host paths. So we remove the class-native setting of
BOTH.

[1] https://cmake.org/cmake/help/latest/variable/CMAKE_FIND_ROOT_PATH_MODE_PROGRAM.html

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>