Update flatMap documentation and remove unused code towards post-JIT semantics

[kushti]

While langspec.md is stating

Function `f` is constrained to be of the form `x => x.someProperty`, otherwise it is illegal.

for flatMap on collections, there is no such restrictions since JIT.

To clean outdated things up here in regards with flatMap semantics post-JIT change, following steps are needed:

• update langspec.md
• update corresponding comment in the code ( Update flatMap documentation for Interpreter V2/Ergo 5.0 #954 )
• remove checkValidFlatmap used in tests only now https://github.com/ScorexFoundation/sigmastate-interpreter/blob/efa68e7079393d70d25d43dd63dfba9a18d03415/data/shared/src/main/scala/sigma/ast/methods.scala#L781