chore(common): remove prepublish step from package.json

Given we've just run a build, this (a) seems unnecessary, and (b) seems
to go wrong in some circumstances anyway, e.g.
https://build.palaso.org/buildConfiguration/Keyman_Developer_Release/421326?buildTab=log&focusLine=19936&linesState=18839&logView=flowAware:

```
07:01:29   > @keymanapp/kmc@17.0.205-alpha prepublishOnly
07:01:29   > npm run build
07:01:29
07:01:31
07:01:31   > @keymanapp/kmc@17.0.205-alpha build
07:01:31   > tsc -b
07:01:31
07:01:35   ../../../common/web/types/build/src/kmx/kmx.d.ts(1,1): error TS1036: Statements are not allowed in ambient contexts.
07:01:35   ../../../common/web/types/build/src/kmx/kmx.d.ts(1,2): error TS1345: An expression of type 'void' cannot be tested for truthiness.
...
```