fix(gantt): remove gantt splitter double border #3350

Job	Run time
Status check > Unit	1s
CI \| Unit	1s
CI \| Visual	1s
Compile themes / run	32s
Lint scripts / run	55s
Lint styles / run (default)	2m 14s
Lint styles / run (bootstrap)	2m 12s
Render test pages / verify	7s
Lint styles / run (material)	2m 15s
Lint styles / run (classic)	2m 18s
Lint styles / run (nouvelle)	44s
Lint styles / run (fluent)	39s
Render test pages / run	2m 13s
Status check > Lint scripts	1s
Status check > Lint styles	1s
Status check > A11y / prepare	1s
Create screenshots / run (default)	2m 29s
Create screenshots / run (bootstrap)	2m 30s
Create screenshots / run (material)	2m 34s
Create screenshots / run (classic)	2m 36s
Create screenshots / run (nouvelle)	1m 41s
Create screenshots / run (fluent)	2m 24s
Status check > A11y / run-checks (1)	2m 40s
Status check > A11y / run-checks (2)	2m 51s
Status check > A11y / run-checks (3)	2m 40s
Status check > A11y / run-checks (4)	2m 44s
Detect changes	16s
Status check > Visual	1s
	39m 41s

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