Remove $(DEX2OAT) from DEX2OAT_DEPENDENCY when USE_DEX2OAT_DEBUG is set.

Currently, DEX2OATD_DEPENDENCY is created by appending $(DEX2OATD) to
the value of DEX2OAT_DEPENDENCY and then if USE_DEX2OAT_DEBUG is set,
DEX2OAT_DEPENDENCY is set to DEX2OATD_DEPENDENCY, adding a dependency
on $(DEX2OAT) even in configurations where this will not be built.

This change removes DEX2OATD and DEX2OATD_DEPENDENCY entirely to instead
assign dex2oat or dex2oatd to DEX2OAT depending on USE_DEX2OAT_DEBUG,
making it possible to build with these flags set:

USE_DEX2OAT_DEBUG=true
ART_BUILD_HOST_NDEBUG=false
ART_BUILD_HOST_DEBUG=true
ART_BUILD_TARGET_NDEBUG=false
ART_BUILD_TARGET_DEBUG=true
make build-art-host

Change-Id: I4fa75d1b7e9751c73c43ec943fd8446cc231a070