Go to Search -> Replace
Select "Regular expression" under Search mode.
Use ^\s* for "Find what" and leave "Replace with" blank.
Click Replace all
^ means beginning of the line
\s* means any number (even 0) of whitespace characters. Whitespace characters include tab, space, newline, and carriage return.
You can also use the built in feature.
Select everything (Ctrl+A), go to Edit>Blank Operation>Trim Trailing Space.