Remove incorrect STAmount assert:

When computing rates for offers, an STAmount's value can be out of can be out of
range (before canonicalizing). There was an assert that could incorrectly fire
in some cases. This patch removes that assert.

src/ripple/protocol/impl/STAmount.cpp

@@ -227,7 +227,6 @@ STAmount::STAmount (Issue const& issue,
     , mOffset (exponent)
     , mIsNegative (negative)
 {
-    assert(mValue <= std::numeric_limits<std::int64_t>::max());
     canonicalize ();
 }