The Wall Street boom is petering out at US banks.For much of 2020 and through the initial quarter of this year, the largest US banks posted blockbuster results from trading stocks and bonds and advising companies on deals. The Federal Reserve saturated the market with money, companies raced to sell new debt and go public, and traders on Reddit and from large institutions moved those securities quickly. The Banks in the middle of all of those transactions secured the rewards.Now, executives are warning that their market revenue is falling, at least compared with the past year.JPMorgan Chase & Co. Chief Executive James Dimon stated at a financial conference this week that his firm’s trading revenue, both fixed-income and equities, would be over $6 billion in the second quarter. That would be down some 38% from the year-ago period when the bank made a record $9.7 billion.“Still pretty good” is how Mr. Dimon described it. Indeed, $6 billion would have been close to a quarterly record at any time before 2020.Citigroup Inc. Chief Financial Officer Mark Mason warned second-quarter trading revenue will be down at roughly 30% from the prior year, with a drop-off in fixed-income trading.