Merge #13396: Drop unused arith_uint256 ! operator

2acd1d6716 Drop uint 256 not operator (Ben Woosley)

Pull request description:

  All the other operators are integer or bitwise operations, and this is unused
  apart from tests.

  Note attempting to call `!` on `arith_uint256` results in a build error after this change:
  ```
  test/arith_uint256_tests.cpp:201:17: error: invalid argument type 'const arith_uint256' to unary expression
      BOOST_CHECK(!ZeroL);
  ```

Tree-SHA512: 5791b643f426dac9829e9499d678786f1ad294edb2d840879252a1b642bda55941632114f64048660a5991a984aeba49eeb5dfe64ba0a6275cbe7b1c049d7095

src/arith_uint256.h

@@ -64,14 +64,6 @@ public:
 
     explicit base_uint(const std::string& str);
 
-    bool operator!() const
-    {
-        for (int i = 0; i < WIDTH; i++)
-            if (pn[i] != 0)
-                return false;
-        return true;
-    }
-
     const base_uint operator~() const
     {
         base_uint ret;

src/test/arith_uint256_tests.cpp

@@ -198,13 +198,6 @@ BOOST_AUTO_TEST_CASE( shifts ) { // "<<"  ">>"  "<<="  ">>="
 
 BOOST_AUTO_TEST_CASE( unaryOperators ) // !    ~    -
 {
-    BOOST_CHECK(!ZeroL);
-    BOOST_CHECK(!(!OneL));
-    for (unsigned int i = 0; i < 256; ++i)
-        BOOST_CHECK(!(!(OneL<<i)));
-    BOOST_CHECK(!(!R1L));
-    BOOST_CHECK(!(!MaxL));
-
     BOOST_CHECK(~ZeroL == MaxL);
 
     unsigned char TmpArray[32];