Does adding slices of lemons to water reduce its chlorine content?
Yes , a quick and cheap way to reduce chlorine residual is to add ascorbic acid 
 (VITAMIN C), which if added in an equal molar amount to free chlorine would completely eliminate chlorine residual within 1 min. 

I am quoting from ABSTRACT from a paper from, Int. Journal of Engineering Research and Applications , ISSN : 2248-9622, Vol. 3, Issue 5, Sep-Oct 2013, pp.1647-1651.

"Society’s preference for bottled water mainly comes from a dislike of the mildly pungent taste and odor of tap water. Such odor and taste may be caused by chlorine residual (in the form of free chlorine or chloramines), which is maintained in a water distribution system to prevent regrowth of microorganisms and is thus present in tap water.

"Two methods of reducing chlorine residual, boiling tap water and adding ascorbic acid to
tap water, were tested. The results showed that, for free chlorine, a quick and cheap way to reduce chlorine residual is to add ascorbic acid, which if added in an equal molar amount to free chlorine would complete eliminate chlorine residual within 1 min " 


Reduction of Chlorine Residual in Tap Water with Temperature




source : https://www.ijera.com/papers/Vol3_issue5/JN3516471651.pdf

As shown aboce, the chlorine residual decreased faster at higher temperatures. 


Reduction of Free Chlorine and Monochloramine in Tap Water through Boiling


 

source : https://www.ijera.com/papers/Vol3_issue5/JN3516471651.pdf

As shown in Figure 2, both residuals of free chlorine and monochloramine decreased with boiling time, and the residual of free chlorine decreased slightly faster than that of monochloramine.

The boiling time required to completely remove free chlorine was about 33 min.$\ce{^1}$


Effect of Ascorbic Acid on Free Chlorine and Monochloramine


Ascorbic acid added in a molar ratio of 1:1 (free chlorine to ascorbic acid) completely eliminated free chlorine. The DPD indicator(A DPD solution can also be used for the measurement of chlorine residual) did not produce a red color when added within 1 min,indicating complete elemination chlorine residual within 1 min.$\ce{^2}$ 

The reduction of free chlorine by ascorbic acid can be expressed as follows:

$\ce{C6H8O6}$ + $\ce{HOCl}$  →$\ce{C6H6O6}$ +$\ce{HCl}$ + $\ce{H2O}$

Ascorbic acid $\ce{(C6H8O6)}$ is oxidized to dehydroascorbic acid $\ce{(C6H6O6)}$ . Simultaneously, free chlorine $\ce{(HOCl)}$ is reduced to chloride ions (HCl).

In the experiment conducted it was determined that 10 mg of ascorbic acid is required to treat 1 L of tap water. and the quickest way to reduce the concentration of free chlorine in tap water is to add powdered vitamin C (ascorbic acid) tablets in less than 1 min.$\ce{^3}$ 



Thanks to N4v i will be using this cost effective method to remove chlorine from water.(wow).

Reference 


https://www.ijera.com/papers/Vol3_issue5/JN3516471651.pdf
https://www.ijera.com/papers/Vol3_issue5/JN3516471651.pdf
https://www.ijera.com/papers/Vol3_issue5/JN3516471651.pdf