package/grpc: remove obsolete patch to support cross-compiling

package/grpc: remove obsolete patch to support cross-compiling

When cross-compiling grpc, a native tool 'grpc_cpp_plugin' is needed.
Patch '0001-target-build-using-host-plugin.patch' in Buildroot provides a
way to pass the path to this tool via a configure option
'gRPC_NATIVE_CPP_PLUGIN'.

In version 1.20.0, the upstream grpc project added better support for
cross-compiling via commit 0d7a0ded [1], searching for the native
grpc_cpp_plugin via PATH (rather than specifying it as configure option as
our patch was doing).

This change renders the mentioned Buildroot patch obsolete, so remove it.

[1] https://github.com/grpc/grpc/commit/0d7a0ded1cc93bb7f4d69a156b0a69829557cbf2

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Reviewed-by: Michael Nosthoff <buildroot@heine.tech>
Tested-by: Michael Nosthoff <buildroot@heine.tech>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>