Build & config: Remove leftover files from the unsupported "make" build system

* Original commit: espressif/esp-idf@766aa57084

examples/protocols/slip/slip_udp/Makefile

@@ -1,8 +0,0 @@
-#
-# This is a project Makefile. It is assumed the directory this Makefile resides in is a
-# project subdirectory.
-#
-
-PROJECT_NAME := slip_client
-
-include $(IDF_PATH)/make/project.mk

examples/protocols/slip/slip_udp/components/slip_modem/component.mk

@@ -1,3 +0,0 @@
-COMPONENT_ADD_INCLUDEDIRS := include
-
-COMPONENT_SRCDIRS := library

examples/protocols/slip/slip_udp/main/component.mk

@@ -1,4 +0,0 @@
-#
-# "main" pseudo-component makefile.
-#
-# (Uses default behaviour of compiling all source files in directory, adding 'include' to include path.)