utils/fragmented_vector: Remove assignment from std::vector

Assignment from `std::vector` was introduced in redpanda-data#8469.

Now that kafka/protocol/generator can call
kafka::protocol::read_array<fragmented_vector>, this is no
longer needed, avoiding a large copy.

Signed-off-by: Ben Pope <ben@redpanda.com>

src/v/cloud_storage/tests/tx_range_manifest_test.cc

@@ -41,7 +41,17 @@ static remote_manifest_path
 
 using tx_range_t = model::tx_range;
 
-static std::vector<tx_range_t> ranges = {
+template<typename T = int>
+static fragmented_vector<T>
+make_fragmented_vector(std::initializer_list<T> in) {
+    fragmented_vector<T> ret;
+    for (auto& e : in) {
+        ret.push_back(e);
+    }
+    return ret;
+}
+
+static auto ranges = {
   tx_range_t{
     .pid = model::producer_identity(1, 2),
     .first = model::offset(3),
@@ -78,8 +88,7 @@ SEASTAR_THREAD_TEST_CASE(empty_serialization_roundtrip_test) {
 }
 
 SEASTAR_THREAD_TEST_CASE(serialization_roundtrip_test) {
-    fragmented_vector<tx_range_t> tx_ranges;
-    tx_ranges = ranges;
+    fragmented_vector<tx_range_t> tx_ranges = make_fragmented_vector(ranges);
     tx_range_manifest m(segment_path, std::move(tx_ranges));
     auto [is, size] = m.serialize().get();
     iobuf buf;

src/v/utils/fragmented_vector.h

@@ -207,19 +207,6 @@ class fragmented_vector {
      */
     static size_t elements_per_fragment() { return elems_per_frag; }
 
-    /**
-     * Assign from a std::vector.
-     */
-    fragmented_vector& operator=(const std::vector<T>& rhs) noexcept {
-        clear();
-
-        for (auto& e : rhs) {
-            push_back(e);
-        }
-
-        return *this;
-    }
-
     /**
      * Remove all elements from the vector.
      *

src/v/utils/tests/fragmented_vector_test.cc

@@ -335,23 +335,6 @@ BOOST_AUTO_TEST_CASE(fragmented_vector_vector_clear) {
 
     v = make<int, 8>({5, 5, 5, 5});
     BOOST_CHECK_EQUAL(v->size(), 4);
-
-    v.u = std::vector{1, 2, 3};
-    BOOST_CHECK_EQUAL(v->size(), 3);
-}
-
-BOOST_AUTO_TEST_CASE(fragmented_vector_vector_assign) {
-    std::vector vin0{1, 2, 3};
-    std::vector vin1{4, 5};
-
-    checker<int, 8> v;
-    BOOST_CHECK_EQUAL(v, (make({})));
-
-    v.get() = std::vector{1};
-    BOOST_CHECK_EQUAL(v, (make({1})));
-
-    v.get() = std::vector{2, 3, 4};
-    BOOST_CHECK_EQUAL(v, (make({2, 3, 4})));
 }
 
 BOOST_AUTO_TEST_CASE(fragmented_vector_pop_back_n) {