shared_ptr: remove deprecated lw_shared_ptr assignment operator

The assignment operator was deprecated in 8038c13 ("shared_ptr:
deprecate lw_shared_ptr operator=(T&&)"), more than a year ago. It
is safe to remove it now.

Closes #2456

include/seastar/core/shared_ptr.hh

@@ -342,12 +342,6 @@ public:
     lw_shared_ptr& operator=(std::nullptr_t) noexcept {
         return *this = lw_shared_ptr();
     }
-    [[deprecated("call make_lw_shared<> and assign the result instead")]]
-    lw_shared_ptr& operator=(T&& x) noexcept {
-        this->~lw_shared_ptr();
-        new (this) lw_shared_ptr(make_lw_shared<T>(std::move(x)));
-        return *this;
-    }
 
     T& operator*() const noexcept { return *accessors<T>::to_value(_p); }
     T* operator->() const noexcept { return accessors<T>::to_value(_p); }