Revert "Refactor Endpoint API"

include/grpc/support/slice_buffer.h

src/core/httpcli/httpcli.c

src/core/iomgr/endpoint.c

src/core/iomgr/endpoint.h

src/core/iomgr/tcp_posix.c

src/core/iomgr/tcp_windows.c

src/core/security/secure_endpoint.c

src/core/security/secure_transport_setup.c

src/core/support/slice_buffer.c

src/core/transport/chttp2/internal.h

src/core/transport/chttp2/writing.c

src/core/transport/chttp2_transport.c

test/core/bad_client/bad_client.c

test/core/iomgr/endpoint_tests.c

test/core/iomgr/tcp_posix_test.c

test/core/security/secure_endpoint_test.c

test/core/util/port_posix.c

test/core/util/port_windows.c