runtime: delete unused cbctxts variable on windows assembly

runtime·cbctxts has been unused since CL 258938, but it was left over.

Change-Id: I374ad26e668a36994e41f5d17593b33090bdc644
Reviewed-on: https://go-review.googlesource.com/c/go/+/463119
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/runtime/sys_windows_386.s

@@ -111,8 +111,6 @@ TEXT runtime·lastcontinuetramp(SB),NOSPLIT,$0-0
 	MOVL	$const_callbackLastVCH, CX
 	JMP	sigtramp<>(SB)
 
-GLOBL runtime·cbctxts(SB), NOPTR, $4
-
 TEXT runtime·callbackasm1(SB),NOSPLIT,$0
   	MOVL	0(SP), AX	// will use to find our callback context
 

src/runtime/sys_windows_amd64.s

@@ -140,8 +140,6 @@ TEXT runtime·lastcontinuetramp(SB),NOSPLIT|NOFRAME,$0-0
 	MOVQ	$const_callbackLastVCH, DX
 	JMP	sigtramp<>(SB)
 
-GLOBL runtime·cbctxts(SB), NOPTR, $8
-
 TEXT runtime·callbackasm1(SB),NOSPLIT|NOFRAME,$0
 	// Construct args vector for cgocallback().
 	// By windows/amd64 calling convention first 4 args are in CX, DX, R8, R9

src/runtime/sys_windows_arm.s

@@ -131,8 +131,6 @@ TEXT runtime·lastcontinuetramp(SB),NOSPLIT|NOFRAME,$0
 	MOVW	$const_callbackLastVCH, R1
 	B	sigtramp<>(SB)
 
-GLOBL runtime·cbctxts(SB), NOPTR, $4
-
 TEXT runtime·callbackasm1(SB),NOSPLIT|NOFRAME,$0
 	// On entry, the trampoline in zcallback_windows_arm.s left
 	// the callback index in R12 (which is volatile in the C ABI).

src/runtime/sys_windows_arm64.s

@@ -155,8 +155,6 @@ TEXT runtime·lastcontinuetramp(SB),NOSPLIT|NOFRAME,$0
 	MOVD	$const_callbackLastVCH, R1
 	B	sigtramp<>(SB)
 
-GLOBL runtime·cbctxts(SB), NOPTR, $4
-
 TEXT runtime·callbackasm1(SB),NOSPLIT,$208-0
 	NO_LOCAL_POINTERS