LUDOC-449 misc: Use .png files instead of .svg

LUDOC-449 misc: Use .png files instead of .svg

Some browsers are not displaying figures in the manual that are
using .svg format.  Since all other images are using .png format,
this patch addresses the few svg images that are in the figures
directory:
figures/File_Write.svg:  duplicate png exists; Unused - removed
figures/HSM_copytool.svg:  Converted to png and removed
figures/MDTs_Failover.svg: duplicate png exists;  Unused - removed
figures/Metadata_File.svg: duplicate png exists;  Unused - removed
figures/TBF_policy.svg:  Converted to png and removed

Signed-off-by: Joseph Gmitter <jgmitter@whamcloud.com>
Change-Id: I18132933d9bb51a73c084bf42cf2f59355cbef4e
Reviewed-on: https://review.whamcloud.com/35558
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>