Well there's your answer then. 1979 there were a few seats the Liberals didn't stand in (actually quite a lot in Scotland and Wales but only a handful in England). As well as the Salfords there were two in Birmingham (Handsworth being another straight Labour/Conservative fight but in Sparkbrook a Communist candidate held the Jiggesrist vote down to 97.4%). I can't remember the others, but you actually asked 'when' was the last time as opposed to 'where' so the answer is 1979 anyway..
I can though. The other English seats where the Liberals did not stand in 1979 were Manchester Openshaw, Walsall South, West Bromwich West, Warley East, Ormskirk, and Dudley West (a straight fight which also resulted in a Conservative gain). The National Front contested the first four of these (there was also a Communist candidate in Manchester Openshaw) and the Workers' Revolutionary Party contested Ormskirk.
Birmingham also generated the lowest Liberal vote in England where there was a Liberal candidate-Olive Griffiths only polled 4.6% as the Liberal candidate in closely fought Birmingham Perry Barr that year.
To show face? I think the Lib Dems did the same thing in Ealing and Acton
No they didn't, not at all. Indeed the LD candidate Jon Ball caused considerable amusement this year when he declared he would win the seat (although they were in contention there in 2010). You might be thinking of the independent candidate who declared his support for Rupa Huq when she was first elected in 2015 - indeed, her win then was so narrow that it could have made a real difference.