tdf#126268: first byte is not used for the sign symbol

tdf#126268: first byte is not used for the sign symbol

This code is just broken, negative numbers and positive
numbers above 25 are incorrectly migrated if using decimal.
E.g:
-1 -> 24,6
26 -> -6527'6

thus, delete it and pass the 8 bytes as they are to
lcl_double_dabble.

Note: this only fixes the positive numbers,
-1 still returns 24,6

Code introduced in 6039d2463b4cc503b963811b3b1471de3291ee09

Change-Id: Id0e249e47a91b274a247fb7e8b71353114149650
Reviewed-on: https://gerrit.libreoffice.org/75375
Tested-by: Jenkins
Reviewed-by: Lionel Elie Mamane <lionel@mamane.lu>
Reviewed-by: Xisco Faulí <xiscofauli@libreoffice.org>