Remove -DBSL_OVERRIDES_STD from bsl.defs

BSL_OVERRIDES_STD imports bsl headers to the std namespace and should be
used only internally for backwards compatibility with the existing
source code. We are removing BSL_OVERRIDES_STD from bsl.defs so that it
is no longer the default.
[APPROVAL] review/remove-bsl-overrides-std-from-oss-drqs-56739751 to master