Remove feature(unchecked_math) from miri

Author: scottmcm

Diff for: src/tools/miri/tests/fail/intrinsics/unchecked_add1.rs

@@ -1,5 +1,3 @@
-#![feature(unchecked_math)]
-
 fn main() {
     // MAX overflow
     let _val = unsafe { 40000u16.unchecked_add(30000) }; //~ ERROR: overflow executing `unchecked_add`

Diff for: src/tools/miri/tests/fail/intrinsics/unchecked_add2.rs

@@ -1,5 +1,3 @@
-#![feature(unchecked_math)]
-
 fn main() {
     // MIN overflow
     let _val = unsafe { (-30000i16).unchecked_add(-8000) }; //~ ERROR: overflow executing `unchecked_add`

Diff for: src/tools/miri/tests/fail/intrinsics/unchecked_mul1.rs

@@ -1,4 +1,3 @@
-#![feature(unchecked_math)]
 fn main() {
     // MAX overflow
     let _val = unsafe { 300u16.unchecked_mul(250u16) }; //~ ERROR: overflow executing `unchecked_mul`

Diff for: src/tools/miri/tests/fail/intrinsics/unchecked_mul2.rs

@@ -1,4 +1,3 @@
-#![feature(unchecked_math)]
 fn main() {
     // MIN overflow
     let _val = unsafe { 1_000_000_000i32.unchecked_mul(-4) }; //~ ERROR: overflow executing `unchecked_mul`

Diff for: src/tools/miri/tests/fail/intrinsics/unchecked_sub1.rs

@@ -1,4 +1,3 @@
-#![feature(unchecked_math)]
 fn main() {
     // MIN overflow
     let _val = unsafe { 14u32.unchecked_sub(22) }; //~ ERROR: overflow executing `unchecked_sub`

Diff for: src/tools/miri/tests/fail/intrinsics/unchecked_sub2.rs

@@ -1,4 +1,3 @@
-#![feature(unchecked_math)]
 fn main() {
     // MAX overflow
     let _val = unsafe { 30000i16.unchecked_sub(-7000) }; //~ ERROR: overflow executing `unchecked_sub`