[mlir][doc] Remove duplicate syntax formats (#73343)

Some operations defined their syntax both in the documentation and via
`assemblyFormat`. This leads to two `Syntax:` subheadings in the
documentation, see for example the documentation for
[`arith.maximumf`](https://mlir.llvm.org/docs/Dialects/ArithOps/#arithmaximumf-arithmaximumfop).
Since the `assemblyFormat` is used to generate the actual parsers and
printer implementations, this PR suggest to remove the manual syntax
descriptions.