incompatible_disable_static_cc_toolchains: Remove @bazel_tools//tools/cpp:default-toolchain and friends

[scentini]

Flag: --incompatible_disable_static_cc_toolchains
Available since: 0.28
Will be flipped: 1.0

Currently we have a @bazel_tools//tools/cpp:default-toolchain cc_toolchain_suite, along with multiple cc_toolchain targets that Bazel doesn't use.

Users use it by specifying --host_crosstool_top=@bazel_tools//tools/cpp:default-toolchain and/or --crosstool_top=@bazel_tools//tools/cpp:default-toolchain.

The incompatible_disable_static_cc_toolchains flag will disable using the @bazel_tools//tools/cpp:default-toolchain toolchain suite, as well as all the individual cc_toolchains under tools/cpp/BUILD

Migration

Users should use --host_crosstool_top=@bazel_tools//tools/cpp:toolchain and/or --crosstool_top=@bazel_tools//tools/cpp:toolchain instead, which uses the auto-configuration mechanism. Users can also omit the --crosstool_top and --host_crosstool_top options altogether, as they point to @bazel_tools//tools/cpp:toolchain by default.

[laurentlb]

If it's going to be flipped for 1.0, please add the label breaking-change-1.0.