[FIX] web_editor: remove copy paste unwanted newline

Before this commit:

- When you try to paste text beside a text which is in code formate it
  puts the new text in new line with smaller font.

- When we try to paste text in code formate in new line then it will
  give a blank space below the pasted text

After this commit:

- when you try to paste text beside a text which is in code formate it
  puts the pasted text in new line with original fontsize.

- When we paste the text in code formate in new line then it will not give
  a empty line below it,

Task-2683455

closes odoo/odoo#105939

X-original-commit: 449cacfe
Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>