docs: drop duplicate `21.1` branch name from release calendar

Part-of: <mesa/mesa!11892>

docs/release-calendar.csv

-21.0,2021-05-05,21.0.4,Dylan Baker,This is the last planned release of the 21.0.x series.
-21.1,2021-07-28,21.1.6,Eric Engestrom,
-21.1,2021-08-11,21.1.7,Eric Engestrom,
-21.1,2021-08-25,21.1.8,Eric Engestrom,
-21.2,2021-07-21,21.2.0-rc2,Dylan Baker
-,2021-07-28,21.2.0-rc3,Dylan Baker
-,2021-08-04,21.2.0-rc4,Dylan Baker,Or 21.2.0 final.
+21.0,2021-05-05,21.0.4,Dylan Baker,This is the last planned release of the 21.0.x series.
+21.1,2021-07-28,21.1.6,Eric Engestrom,
+,2021-08-11,21.1.7,Eric Engestrom,
+,2021-08-25,21.1.8,Eric Engestrom,
+21.2,2021-07-21,21.2.0-rc2,Dylan Baker
+,2021-07-28,21.2.0-rc3,Dylan Baker
+,2021-08-04,21.2.0-rc4,Dylan Baker,Or 21.2.0 final.