Utils: Remove operator[]

Because the operator is always accessing data which has a branch is can be
much slower than iterators. SmallString is optimized for performance so
it would be quite misleading to have a non performing access operator.

Change-Id: Id882c3c12508afa393ce0766bbb680690a193c95
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>

src/libs/utils/smallstring.h

@@ -177,16 +177,6 @@ public:
         swap(first.m_data, second.m_data);
     }
 
-    char &operator[](size_type index)
-    {
-        return at(index);
-    }
-
-    const char &operator[](size_type index) const
-    {
-        return at(index);
-    }
-
     QByteArray toQByteArray() const noexcept
     {
         return QByteArray(data(), int(size()));