remove plan9 test coverage

This regressed; the plan9 linker code is crashing when trying to build
compiler-rt and we have no active plan9 maintainers involved in the Zig
project. Anyone is welcome to come over and take the role; however, it's
not one of the tier 1, 2, or 3 targets, so we will not be blocking
progress towards 1.0 on plan9.

test/cases/plan9/exit.zig

@@ -1,5 +0,0 @@
-pub fn main() void {}
-
-// run
-// target=x86_64-plan9,aarch64-plan9
-//

test/cases/plan9/hello_world_with_updates.0.zig

@@ -1,28 +0,0 @@
-pub fn main() void {
-    const str = "Hello World!\n";
-    asm volatile (
-        \\push $0
-        \\push %%r10
-        \\push %%r11
-        \\push $1
-        \\push $0
-        \\syscall
-        \\pop %%r11
-        \\pop %%r11
-        \\pop %%r11
-        \\pop %%r11
-        \\pop %%r11
-        :
-        // pwrite
-        : [syscall_number] "{rbp}" (51),
-          [hey] "{r11}" (@ptrToInt(str)),
-          [strlen] "{r10}" (str.len),
-        : "rcx", "rbp", "r11", "memory"
-    );
-}
-
-// run
-// target=x86_64-plan9
-//
-// Hello World
-//

test/cases/plan9/hello_world_with_updates.1.zig

@@ -1,10 +0,0 @@
-const std = @import("std");
-pub fn main() void {
-    const str = "Hello World!\n";
-    _ = std.os.plan9.pwrite(1, str, str.len, 0);
-}
-
-// run
-//
-// Hello World
-//