[diag] move factory diags into core (#4010)

- Simplifies make files by not having to deal with separate library.
- Combine factory diags source into single source file.

examples/apps/cli/Makefile.am

@@ -57,12 +57,6 @@ LDADD_COMMON                                                          += \
     $(NULL)
 endif # OPENTHREAD_ENABLE_BUILTIN_MBEDTLS
 
-if OPENTHREAD_ENABLE_DIAG
-LDADD_COMMON                                                          += \
-    $(top_builddir)/src/diag/libopenthread-diag.a                        \
-    $(NULL)
-endif # OPENTHREAD_ENABLE_DIAG
-
 if OPENTHREAD_ENABLE_FTD
 bin_PROGRAMS                                                          += \
     ot-cli-ftd                                                           \

examples/apps/cli/main.c

@@ -104,10 +104,6 @@ pseudo_reset:
 
     otCliUartInit(instance);
 
-#if OPENTHREAD_ENABLE_DIAG
-    otDiagInit(instance);
-#endif
-
     while (!otSysPseudoResetWasRequested())
     {
         otTaskletsProcess(instance);