Clean up unused flags for LLD.

Since Android only uses LLD, we shouldn't be default adding flags that
just get stripped by a later pass such as:

-Wl,-m,aarch64_elf64_le_vec
-fuse-ld=gold

Remaining query for `-fuse-ld=gold`:
https://source.corp.google.com/search?q=%5C-fuse-ld%3Dgold%20file:.*%5C.(bp%7Cmk%7Cgo)&sq=package:%5Eandroid$
Remaining query for `elf64_le_vec`:
https://source.corp.google.com/search?q=elf64_le_vec%20file:.*%5C.(bp%7Cmk%7Cgo)&sq=package:%5Eandroid$

Bug: http://b/148819762
Test: Built aosp_blueline
Change-Id: I4e064a1e3e0c54721f3f08af8d717f28f6d348f8

cc/config/arm64_device.go

@@ -37,10 +37,8 @@ var (
 	}
 
 	arm64Ldflags = []string{
-		"-Wl,-m,aarch64_elf64_le_vec",
 		"-Wl,--hash-style=gnu",
 		"-Wl,-z,separate-code",
-		"-fuse-ld=gold",
 		"-Wl,--icf=safe",
 	}