You have not specified the version, I'm assuming you are using Powershell v3.
Try this:
$path = "C:\Users\abc\Desktop\File\abc.txt"
(Get-Content $path -Raw).Replace("`r`n","`n") | Set-Content $path -Force
Editor's note: As mike z points out in the comments, Set-Content appends a trailing CRLF, which is undesired. Verify with: 'hi' > t.txt; (Get-Content -Raw t.txt).Replace("`r`n","`n") | Set-Content t.txt; (Get-Content -Raw t.txt).EndsWith("`r`n"), which yields $True.
Note this loads the whole file in memory, so you might want a different solution if you want to process huge files.
UPDATE
This might work for v2 (sorry nowhere to test):
$in = "C:\Users\abc\Desktop\File\abc.txt"
$out = "C:\Users\abc\Desktop\File\abc-out.txt"
(Get-Content $in) -join "`n" > $out
Editor's note: Note that this solution (now) writes to a different file and is therefore not equivalent to the (still flawed) v3 solution. (A different file is targeted to avoid the pitfall Ansgar Wiechers points out in the comments: using > truncates the target file before execution begins). More importantly, though: this solution too appends a trailing CRLF, which may be undesired. Verify with 'hi' > t.txt; (Get-Content t.txt) -join "`n" > t.NEW.txt; [io.file]::ReadAllText((Convert-Path t.NEW.txt)).endswith("`r`n"), which yields $True.
Same reservation about being loaded to memory though.