powerpc/64: Remove unused generic_secondary_thread_init()

The last caller was removed in 2014 in commit fb5a515704 ("powerpc:
Remove platforms/wsp and associated pieces").

As Jordan noticed even though there are no callers, the code above in
fsl_secondary_thread_init() falls through into
generic_secondary_thread_init(). So we can remove the _GLOBAL but not
the body of the function.

However because fsl_secondary_thread_init() is inside #ifdef
CONFIG_PPC_BOOK3E, we can never reach the body of
generic_secondary_thread_init() unless CONFIG_PPC_BOOK3E is enabled,
so we can wrap the whole thing in a single #ifdef.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200819015704.1976364-1-mpe@ellerman.id.au

arch/powerpc/kernel/head_64.S

@@ -300,9 +300,6 @@ _GLOBAL(fsl_secondary_thread_init)
 	rlwimi	r3, r3, 30, 2, 30
 	mtspr	SPRN_PIR, r3
 1:
-#endif
-
-_GLOBAL(generic_secondary_thread_init)
 	mr	r24,r3
 
 	/* turn on 64-bit mode */
@@ -312,13 +309,13 @@ _GLOBAL(generic_secondary_thread_init)
 	bl	relative_toc
 	tovirt(r2,r2)
 
-#ifdef CONFIG_PPC_BOOK3E
 	/* Book3E initialization */
 	mr	r3,r24
 	bl	book3e_secondary_thread_init
-#endif
 	b	generic_secondary_common_init
 
+#endif /* CONFIG_PPC_BOOK3E */
+
 /*
  * On pSeries and most other platforms, secondary processors spin
  * in the following code.