I had the same problem and since couldn't find a direct solution I came up with this workaround. I hope it will be helpful for anyone else with the same problem.
You can modify your SRT subtitle file with this macro in MS Word. Click "view macros" > "create macro" and paste the following script:
Sub MergAll2closeSubLines()
'
' MergAll2closeSubLines Macro
'
'
Do While Selection.Find.Execute = True
Selection.Find.ClearFormatting
With Selection.Find
.Text = "??:??:??,??? ??? ??:??:??,???"
.Replacement.Text = ""
.Forward = True
'
' change (.Wrap = wdFindContinue) to (.Wrap = wdFindStop)
'
.Wrap = wdFindStop
.MatchWildcards = True
End With
'
' tweak this section based on the formating of subtitle
'
Selection.Find.Execute
Selection.EndKey Unit:=wdLine
Selection.MoveLeft Unit:=wdWord, Count:=7, Extend:=wdExtend
Selection.Copy
Selection.MoveUp Unit:=wdLine, Count:=2, Extend:=wdExtend
Selection.Delete Unit:=wdCharacter, Count:=1
Selection.Delete Unit:=wdCharacter, Count:=1
Application.Browser.Previous
Selection.EndKey Unit:=wdLine
Selection.MoveLeft Unit:=wdWord, Count:=7, Extend:=wdExtend
Selection.PasteAndFormat (wdFormatSurroundingFormattingWithEmphasis)
Loop
End Sub
It basically finds and copies the ending time stamp of every other line, deletes that line's ending time and pastes that in the place of previous line's ending time.
This will modify your subtitle in a few seconds.