[clang][Sema] Remove dead diagnostic for loss of __unaligned qualifier

D120936 has made the loss of `__unaligned` qualifier NOT a bad-conversion.
Because of this, the bad-conversion note about the loss of this qualifier does not take effect.
e.g.
```
void foo(int *ptr);

void func(const __unaligned int *var) { foo(var); }
```
BEFORE this patch:
```
source.cpp:3:41: error: no matching function for call to 'foo'
    3 | void func(const __unaligned int *var) { foo(var); }
      |                                         ^~~
source.cpp:1:6: note: candidate function not viable: 1st argument ('const __unaligned int *') would lose __unaligned qualifier
    1 | void foo(int *ptr);
      |      ^
    2 |
    3 | void func(const __unaligned int *var) { foo(var); }
      |                                             ~~~
```
AFTER this patch:
```
source.cpp:3:41: error: no matching function for call to 'foo'
    3 | void func(const __unaligned int *var) { foo(var); }
      |                                         ^~~
source.cpp:1:6: note: candidate function not viable: 1st argument ('const __unaligned int *') would lose const qualifier
    1 | void foo(int *ptr);
      |      ^
    2 |
    3 | void func(const __unaligned int *var) { foo(var); }
      |                                             ~~~
```
Please note the different mentions of `__unaligned` and `const` in notes.

Reviewed By: cjdb, rnk
Differential Revision: https://reviews.llvm.org/D153690