text: Title: make `remove_stopwords()` behavior more consistent
  Body: (triggered by SO question: https://stackoverflow.com/questions/67944732/using-my-own-stopword-list-with-gensim-corpora-textcorpus-textcorpus/67951592#67951592)
  
  Gensim has two `remove_stopwords()` functions with similar, but slightly-different behavior that risks confusing users. 
  
  `gensim.parsing.preprocessing.remove_stopwords` takes a space-delimited string, and always consults the *current* value of `gensim.parsing.preprocessing.STOPWORDS` (including any user reassignments). 
  
  By contrast, `gensim.corpora.textcorpus.remove_stopwords` takes a list-of-tokens, allows the specification of an alternate list-of-stopwords, but if allowed to use its 'default' stopwords, uses only the value of `gensim.parsing.preprocessing.STOPWORDS` that was captured when the function was defined (which could miss later user redefinitions).
  
  Avoiding the reuse of identical function names that take different argument-types would help avoid user/code-reviewer confusion. Also, capturing the value of a variable that might change leads to confusing function-behavior: the token version could just as easily consult `gensim.parsing.preprocessing.STOPWORDS` on each call (as the string version already does). 