chore: revert decimals are always strings on hana (#700)

It's decided that the prerelase of cds 8 will not switch the default of
`cds.env.features.string_decimals`.
In order to not lose precision on HANA with the prerelease, we have to
remove the interpretation of the flag.
Maybe we should rename the flag in the future to be sqlite specific.

hana/lib/HANAService.js

@@ -1093,9 +1093,8 @@ class HANAService extends SQLService {
       Vector: e => `TO_NVARCHAR(${e})`,
       // Reading int64 as string to not loose precision
       Int64: expr => `TO_NVARCHAR(${expr})`,
-      // REVISIT: always cast to string in next major
       // Reading decimal as string to not loose precision
-      Decimal: cds.env.features.string_decimals ? expr => `TO_NVARCHAR(${expr})` : undefined,
+      Decimal: expr => `TO_NVARCHAR(${expr})`,
 
       // HANA types
       'cds.hana.ST_POINT': e => `(SELECT NEW ST_POINT(TO_NVARCHAR(${e})).ST_X() as "x", NEW ST_POINT(TO_NVARCHAR(${e})).ST_Y() as "y" FROM DUMMY WHERE (${e}) IS NOT NULL FOR JSON ('format'='no', 'omitnull'='no', 'arraywrap'='no') RETURNS NVARCHAR(2147483647))`,